WO2003013031A1

WO2003013031A1 - Modulator transmitter for high-speed fiberoptic communications

Info

Publication number: WO2003013031A1
Application number: PCT/SE2002/001248
Authority: WO
Inventors: Urban Westergren; Stefan Irmscher; Robert LEWÉN
Original assignee: Optillion Ab
Priority date: 2001-08-01
Filing date: 2002-06-25
Publication date: 2003-02-13
Also published as: SE0102646L; SE523350C2; SE0102646D0

Abstract

The present invention relates to an optoelectronic transmitter for converting an electrical information signal (Se) into an optical signal (So) that represents the same information. The proposed optoelectronic transmitter includes a multi-section optoelectronic modulator. The light and electrical fields thus interact over multiple modulator sections (421a - 421c) of a continuous waveguide (421). A particular drive cirucit (423a a 423c) delivers a phase-aligned and amplified electrical signal (S'e, S''e; S'''e) to each modulator section (421a - 421c9. For a given extinction ration of the modulated optical signal (So), a signal voltage swing can thereby be used, which is substantially lower than according to any known single- or multi-section modulator structure. This, in turn, makes the proposed optoelectonic transmitter well suited for sending data at high-speed bitrates.

Description

Modulator Transmitter for High-Speed Fiberoptic Communications

THE BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates generally to conversion of an electrical signal containing certain information into an optical signal containing the same information. More particularly the invention relates to an optoelectronic transmitter according to the preamble of claim 1 and a method of converting an electric signal into an optical signal according to the preamble of claim 1 1 .

Today, optoelectronic transmitters are used for high-speed fiberoptic communications, primarily according to a binary signal format. An electrical input signal, having either a high or a low voltage/current state, is thereby most commonly transformed by the optoelectronic transmitter into a corresponding optical signal, having either a high or a low optical power state. In its simplest form, the optoelectronic transmitter comprises an electronic drive circuit and an optoelectronic device. The electronic drive circuit is typically a semiconductor integrated circuit, i.a. including transistors. The optoelectronic device contains either a semiconductor laser or an optical modulator.

The highest rate at which an optoelectronic transmitter can deliver an information signal is limited both by the frequency behavior of the electronic drive circuit and the optoelectronic device. Moreover, the maximum attainable power of the optical signal depends on the speed of the optoelectronic transmitter. In order to produce an optical signal for transmission via, for instance an optical fiber, the drive circuit and the optoelectronic device must together provide a sufficient bandwidth and extinction ratio (ER) between the optical power levels that represent the high state and the low state respectively. Minimum ER-values are normally specified for each commercial standard, such as the SONET- (synchronous optical network) or the Ethernet standard (IEEE-802.3).

The transistors in the electronic drive circuit as well as in the optoelectronic device impose limitations on the transmitter's working frequency. These limitations are due to a combination of internal charging and transit times.

The transistors in both devices place further limitations on the highest possible ER. On one hand, the risk of avalanche break- down restricts the maximum voltage that the transistors can handle. On the other hand, the optoelectronic device requires a minimum voltage to provide a sufficient ER. The maximum operation speed of any transistor, bipolar or FET, in any given semiconductor material is determined by the transistor's geo- metrical extensions. Increased speed generally requires smaller sizes, which in turn, reduce the avalanche breakdown voltages for the components.

Consequently, a compromise must always be made between speed and voltage for any transistor of a specific material. This compromise is usually referred to as the Johnson limit.

When reducing the dimensions of light modulators, such as electroabsorption modulators (EAM) or traveling wave electroab- sorption modulators (TWEAM), in order to increase their respective maximum operation speed, the required voltage for a certain ER will also increase. This is due to the fact that the size reduction shortens the distance over which the light field and the electric field can interact.

Therefore, a combined electronic drive circuit and optoelectronic device will be limited with respect to a combination of speed and extinction ratio. It is namely necessary to settle a voltage compromise between the electronic drive circuit and the optoelectronic device. There are two fundamentally different, however rather straightforward approaches to circumvent the problem of the voltage connection between the electronic drive circuit and the optoelectronic device. Either the semiconductor material system for the electronic drive circuit may be changed, such that the transistors have a larger bandgap and thus can handle larger voltages before the avalanche breakdown occurs, or the optoelectronic device may be improved to provide a larger bandwidth already at a relatively low drive voltage.

Approach 1 : Common high-speed transistors today include collector or drain regions of silicon (e.g. complementary metal- oxide semiconductor (CMOS) transistors and SiGe bipolar transistors) or InGaAs (e.g. in single heterojunction bipolar transistors (SHBT)). CMOS circuits, where the transistors have cutoff frequencies in the order of 100 GHz and typical sizes of less than 0.1 Oμm, are regularly limited to supply voltages below 1 .5V. Silicon bipolar transistors having cutoff frequencies above 100 GHz are expected to have collector-emitter breakdown voltages below 2.0V, and the even faster InP-based SHBTs with cutoff frequencies above 200 GHz have breakdown voltages slightly above 1V. However in research environments, bipolar transistors have been demonstrated with cutoff frequencies of 200-300 GHz and breakdown voltages in the order of 5V.

Approach 2: For TWEAMs, the bandwidth to drive voltage relationship may be improved by shortening the length of the modulator section. The table below exemplifies some typical state-of-the-art values in this respect.

Length (μm) 3dB-BW (GHz) ER (dB for 1V) 950 14 40 450 43 22 250 67 12 Even if substantial improvements are made along either or both the above approaches, a voltage compromise must still always be made at one point.

The prior art also includes various alternative approaches to accomplish efficient high-speed fiberoptic communications modulators. For instance, multiple consecutive modulator stages may be employed for generating the optical pulses.

G. L. Li et al. give a first example in the article "Ultrahigh-Speed Travelling-Wave Electroabsorption Modulator - Design and Analysis", IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 7, pp 1 177 - 1 183, July 1999. Primarily, the article describes a circuit model for TWEAMs. Nevertheless, the document also presents a passive version of a modulator having several cascaded sections. The optical waveguide here includes relatively long passive parts between rather short modulating sections. Although the design aims at matching velocity/delay between electrical and optical field at standard input impedance (of 50 Ohms), it fails to achieve the necessary reduction of the drive voltage.

V. Kaman et al., "Integrated Tandem Electroabsorption Modulators for High-Speed OTDM Applications", IEEE Photonic Technology Letters, Vol. 12, No. 1 1 , pp 1471 - 1473, November 2000 disclose an optical short pulse generator- and demultiplexer solution for optical time division multiplexed (OTDM) systems operating at bitrates above 100 Gb/s. However, no actual modulator structure is proposed. More important, the OTDM signals require an RZ-format (RZ = return to zero), which is less bandwidth efficient than the more frequently used NRZ- format (NRZ = non-return to zero) and is therefore not as commercially attractive.

The US-patent 5 798 856 discloses an optical pulse generator, which includes two or more cascaded EAMs. At least one modulator in a first set of modulators produces a sequence of short optical pulses. A following modulator may then gate these optical pulses on basis of an information signal, such that an output optical signal represents this information. The document also shows a design example where a laser diode is integrated on the same substrate structure as the modulators. Nevertheless, the proposed designs all demand a comparatively large voltage amplitude to represent the information signal. This, in turn, results in large losses and problems related to power dissipation. Furthermore, the generated optical signal is of the RZ-format, which as mentioned above, utilizes the bandwidth relatively poorly.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an efficient optoelectronic transmitter solution, which alleviates the problems above and thus offers a substantial improvement of the relationship between the transmitter's bandwidth and extinction ratio.

According to one aspect of the invention the object is achieved by an optoelectronic transmitter as initially described, which is characterized in that it comprises an electrical transmission line for receiving the primary electrical signal. The optoelectronic transmitter also includes at least one second drive circuit for receiving a delayed version of the primary electrical signal via the electrical transmission line. In response to the delayed signal, the at least one second drive circuit produces at least one second amplified electrical signal. Furthermore, a continuous optical waveguide is included in the optoelectronic transmitter. The optical waveguide receives the continuous wave optical signal, the first amplified electrical signal, the at least one second amplified electrical signal and produces in response to these signals, a resulting optical signal, which contains the same information as the electrical signal. The one or more second amplified electrical signals enter the continuous optical waveguide such that they are aligned in phase with the already modulated optical signal therein, which results from the first amplified electrical signal.

According to a preferred embodiment of the invention, the continuous optical waveguide includes one modulator section for each of the drive circuits. Each of these modulator sections receives a respective amplified electrical signal. Moreover, they each receive a respective optical signal via the optical waveguide. In response to the received signals, each modulator section then produces, a particular modulated optical signal. The resulting optical signal represents an aggregation of these modulated optical signals.

According to another preferred embodiment of the invention, the electrical transmission line includes a delay element between the first drive circuit and each of the at least one second drive circuit. Each such delay element imposes a particular delay on the electrical signal, which is equivalent to a delay that each of the modulator sections inflict on the modulated optical signal. Thus, the electrical signal(s) from the at least one second drive circuit is/are aligned in phase with the first modulated optical signal (as well as with each other).

According to yet another preferred embodiment of the invention, a first end of the electrical transmission line receives the primary electrical signal and a termination resistor in a second end of the transmission line terminates any remaining energy in the primary electrical signal. Undesired standing waves are thereby avoided in the transmission line.

According to another aspect of the invention the object is achieved by a method of converting an electrical signal into an optical signal, as initially described, which is characterized by the following procedure steps; producing a first delayed electrical signal by delaying the primary electrical signal in proportion to a processing delay in producing the first modulated optical signal, such that an amplified version of the first delayed electrical signal is aligned in phase with the first modulated optical signal, amplifying the first delayed electrical signal into a second amplified electrical signal, modulating the first modulated optical signal with respect to the second amplified electrical signal into a second modulated optical signal, and forming a resulting optical signal by aggregating at least the first modulated optical signal and the second modulated optical signal.

According to a preferred embodiment of the invention, the proposed method comprises the further steps of; producing at least one second delayed electrical signal by delaying the primary electrical signal in proportion to a processing delay in producing the second modulated optical signal such that an amplified version of the second delayed electrical signal is aligned in phase with the second modulated optical signal, amplifying the second delayed electrical signal into a third amplified electrical signal, modulating the second modulated optical signal with respect to the third amplified electrical signal into a third modulated optical signal, and forming the resulting optical signal by aggregating the first, the second and the third modulated optical signal.

According to another preferred embodiment of the invention, the delaying, amplifying modulating and aggregating steps are repeated with respect to at least one additional cycle and the resulting optical signal also includes at least a fourth modulated optical signal.

The proposed multi-section modulator structure offers, for a given extinction ratio, a signal voltage swing being substantially lower than according to any known multi-section modulator structure. This, in turn, guarantees an improved speed and frequency performance irrespective of the modulator type, the transistor technology and semiconductor material. Hence, the transistors of the drive circuits for the modulator can simultaneously have such high upper frequency limit and low breakdown voltage that transistors of any given semiconductor material may be used at high bitrates.

As a further consequence of the invention, optoelectronic transmitter designers are given a larger degree of freedom when choosing the input impedance of the transmitter. This impedance may namely now be chosen to a value being different from the characteristic impedance of the modulator section.

Finally, the invention allows transmission of electrically time- multiplexed optical output signals having an NRZ-format at bitrates above 100 Gb/s. This is superior to any known alternative design.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figure 1 shows a general block diagram over a known optoelectronic transmitter design, Figure 2 shows a circuit diagram over a per se known single- stage optoelectronic transmitter,

Figure 3a shows a graph representing a voltage pulse being delivered by the drive circuit in the optoelectronic transmitter in figure 2, Figure 3b shows a graph that represents a corresponding optical pulse being produced by the optoelectronic transmitter in figure 2,

Figure 4 displays a circuit diagram over an optoelectronic transmitter according to one embodiment of the invention,

Figure 5a shows a graph representing a voltage pulses being delivered by the drive circuits of the optoelectronic transmitter in figure 4,

Figure 5b shows a graph that represents corresponding optical pulses being produced by the optoelectronic transmitter in figure 4, and

Figure 6 illustrates, by means of a flow diagram, a general method according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a general block diagram over a known optoelectronic transmitter 100, which may include an optoelectronic modulator 120 of arbitrary type. An optical source signal generator 1 10, e.g. a semiconductor laser, in the transmitter 100 produces a basic optical signal B₀, which itself lacks an information content. The basic optical signal B₀ may have any format being useful for the particular application. The optoelectronic modulator 120 also receives an electrical signal S_e representing information. By means of the optoelectronic modulator 120, the electrical signal S_e modulates the basic optical signal B₀, such that an outgoing optical signal S₀ is produced that represents the same information as the electrical signal S_e. In most cases, an optical fiber 121 receives the outgoing optical signal S₀ and forwards the signal to its intended end-receivers.

Figure 2 shows a circuit diagram over a per se known single- stage optoelectronic transmitter of TWEAM-type. The basic optical signal B₀ in figure 1 here constitutes a continuous wave optical signal CW₀, which may have been generated by a semiconductor laser. The continuous wave optical signal CW₀ is fed directly into an optical waveguide 221 that also functions as a modulator section. The optical waveguide 221 has a length L, typically in the order of 250-1000 μm, and is terminated by a termination resistor 222 in proximity to its output. In addition to the continuous wave optical signal CW₀, the optical waveguide 221 receives an amplified electrical signal V from a driver circuit 223. The driver circuit 223 in turn receives a primary electrical signal V and produces in response thereto the amplified electrical signal V having corresponding characteristics. Any voltage variations in the input electrical signal V are thus reflected by equivalent variations in the amplified electrical signal V. Particularly, a voltage pulse of a certain magnitude and duration will cause an equivalent pulse in the amplified electrical signal V, for instance having a voltage swing of a magnitude Δ , being initiated at a first point in time ti and ended at a second and somewhat later point in time. Figure 3a displays a graphical representation of such pulse.

When this pulse reaches the optical waveguide 221 the light field of the optical signal CW₀ and the electrical field of the amplified electrical signal V interact. As a result thereof, a modulated optical signal P_opt-v is produced, which represents said pulse of the primary electrical signal V. Figure 3b shows a graph over the latter pulse. However, due to the transit time through the optical waveguide 221 , the optical signal P_opt-v pulse starts a time δ after the primary electrical signal V pulse. The time δ is generally proportional to the length L of the optical waveguide 221 . In any case, the optical signal P_opt-v pulse has a high state of P^ dBm and a low state of P₀ dBm (where the unit "dBm" means decibels relative to 1 milliwatt = 10^~3 watt). The ER for the optical signal P_opt-v thus becomes ER = P^ - P₀, where

ER is expressed in dB.

The invention combines existing technologies for optimizing the frequency behavior of the electronic drive circuit and the optoelectronic device with existing technologies for increasing the attainable power of the optical signal to accomplish a very high transmission rate for the optoelectronic transmitter.

Figure 4 displays a circuit diagram over an optoelectronic transmitter according to one embodiment of the invention. Although the embodiment to be described relates to modulators of TWEAM-type, the invention is equally well applicable to other types of modulators. For example, lumped electroabsorption modulators (EAM) and Mach-Zender modulators may instead be cascaded according to the proposed principle. The former type of modulators have most characteristics in common with TWEAMs, however, their bandwidth is slightly narrower. The latter type of modulators are several factors larger in size and generally require higher drive voltages. The overall working principles nevertheless correspond to that of TWEAMs. In the article "Modeling and Optimization of Traveling-Wave LiNb0₃ Interface Modulators", IEEE Journal of Quantum Electronics, Vol. 27, No. 3, March 1991 , H. Chung et al describe key features of this type of modulators.

Moreover, the invention may be used to improve the performance of any future technologies for developing the frequency behavior of the electronic drive circuit and the optoelectronic device and/or increasing the attainable optical power.

The optoelectronic transmitter in figure 4 includes an optical source signal generator 410, a continuous optical waveguide 421 , an electrical transmission line 424 and a set of drive circuits 423a - 423c.

The optical source signal generator 410 delivers a continuous wave optical signal CW₀ to the optical waveguide 421 . A primary electrical signal S_e that represents information is fed into a first end of the electrical transmission line 424. A first drive circuit 423a also receives the primary electrical signal S_e.

The primary electrical signal S_e may have arbitrary format and thus carry the information according to any type of represen- tation. For example, a voltage below a first level may represent a low state corresponding to a binary "0" and a voltage above a second level may represent a high state corresponding to a binary "1 ". (The first and second levels may, of course, coincide). Hence, a binary sequence "010" can be symbolized by means of a voltage pulse S_e, which initially is low, then increases to a high state and again returns to the low state.

Figure 5a shows a graph of such voltage pulse S'_e after amplification in the first drive circuit 423a. This first amplified electrical signal, i.e. the voltage pulse S'_e, has a voltage swing between a low state and a high state of a magnitude Δ_v/3.

The first amplified electrical signal S'_e is input to the optical fiber 421 , where the light and electrical fields interact as described with reference to the figure 2 above and a first modulated optical signal s'₀ is produced in the waveguide 421 . The continuous wave optical signal CW₀ and the first amplified electrical signal S'_e are presumed to interact over a length L of the waveguide 421 . At the end of this modulator section a first termination resistor 422a is attached. As mentioned earlier, the first modulated optical signal s'₀ will be slightly delayed relative the primary electrical signal S_e to an extent δ that depends on the length L.

A first delay element 424a in the electrical transmission line 424 delays the primary electrical signal S_e to such degree that a first delayed signal S_e ^d1 , after having passed through a second drive circuit 423b, becomes aligned in phase with the first modulated optical signal s'₀. In analogy with the delay δ being dependent on the length L of the first modulator section, the first delay element 424a typically corresponds to a certain length of the electrical transmission line 424. The second drive circuit 423b receives the first delayed signal S_e ^d1 and produces in response thereto a second amplified electrical signal S"_e, which is fed to the continuous optical waveguide 421. The second amplified electrical signal S"_e enters the waveguide 421 such that it is aligned in phase with the first modulated optical signal s'₀. In analogy with the first modulator section, a second termination resistor 422b is attached to the optical wave guide 421 at the end of the section. The light field of an aggregation of the continuous wave optical signal CW₀ and the first modulated optical signal s'₀ interact with the electrical field of the second amplified electrical signal S"_e. As a result thereof, a second modulated optical signal s"₀ is produced in the waveguide 421 .

Analogous to the first delay element 424a, a second delay element 424b in the electrical transmission line 424 delays the primary electrical signal S_e further and to such degree that a second delayed signal S_e ^d2, after having passed through a third drive circuit 423c, becomes aligned in phase with the second modulated optical signal s"₀. The third drive circuit 423c receives the second delayed signal S_e ^d2 and produces in response thereto a third amplified electrical signal S'"_e, which is fed to the continuous optical waveguide 421 .

The third amplified electrical signal S'"_e enters the waveguide 421 such that it is aligned in phase with the second modulated optical signal s"₀ (as well as the first modulated optical signal s'_o). Again, an aggregated light field of the continuous wave optical signal CW₀, the first modulated optical signal s'₀ and the second modulated optical signal s"₀ interact with the electrical field of the third amplified electrical signal S'"_e and a third modulated optical signal s'"₀ is generated in the waveguide 421 . In analogy with the first and second modulator sections, a third termination resistor 422c is also attached to the optical wave guide 421 at the end of the third section.

Furthermore, a termination resistor 425 connected to the opposite end 424c of the electrical transmission line 424 from where the primary electrical signal S_e is fed in, terminates any remaining energy S_e ^d3 in the primary electrical signal S_e. Undesired standing waves are thereby avoided in the transmission line 424.

In principle, the above-described delaying, amplifying modulating and aggregating steps may be repeated an arbitrary number of times. However due to e.g. losses in the electrical transmission line 424, three or four modulator sections may be assumed to be most efficient, at least with respect to bipolar- transistor based driver circuits. The modulator structure exemplified here only includes three modulator sections, and thus, a resulting optical signal S₀ being fed out from the optoelectronic transmitter is an aggregation of the first s'₀, the second s"₀ and the third s'"₀ modulated optical signals.

Moreover, any length interrelationship between the modulator sections 421 a, 421 b and 421 c is conceivable. However, for most implementations an equal length L of the respective sections 421 a - 421 c is optimal. In fact, this assumption has been made in this example, such that the total active length of the continuous optical waveguide 421 becomes 3L.

In order to further elucidate the working principle and the advantages of the optoelectronic transmitter shown in figure 4, the figures 5a and 5b show graphs representing timing and amplitude relationships between the involved signals. Since the resulting optical signal S₀ constitutes an aggregation of the modulated optical signals s'₀, s"₀ and s'"₀, for a given ER, say P - P₀, each modulator section 421 a - 421 c need only receive an input pulse that has a voltage swing between its low and high state of a magnitude Δ_v/3. The figure 5a shows the first amplified electrical signal S'_e pulse as an unbroken line, the second amplified electrical signal S"_e as a dotted line and the third amplified electrical signal S'"_e as a dashed line. The pulses are shifted somewhat in time due to the delays caused by the delay elements 424a and 424b respectively.

The figure 5b shows corresponding optical pulses being produced by the modulator sections 421 a - 421 c and that propagate through the waveguide 421 . Due to the delays in the modulator sections 421 a - 421 c, also these pulses are shifted in time. An unbroken line here represents the first modulated optical signal s'₀, which has an ER of approximately (P^ - Po)/3 (in dBs). Correspondingly, a dotted line represents a sum of the first modulated optical signal s'₀ and the second modulated optical signal s'₀, which has an ER of approximately 2(Pι - Po)/3 (in dBs), and finally a dashed line represents the resulting optical signal S₀ (i.e. a sum of the first s'₀, the second s"₀ and the third s'"₀ modulated optical signals), which has an ER of P^ - P₀ (in dBs).

It can thus be concluded that, in comparison to a single-section TWEAM having a particular ER, each drive circuit of an analo- gous multi-section modulator can have a relatively low signal voltage swing to the respective modulator sections, typically half or less, depending on the number of modulator sections. The overall ER still becomes sufficiently high, since the resulting optical signal S₀ is an aggregation of the individual optical signals s'₀, s"₀ and s'"₀ being output from all the modulator sections.

Preferably, the proposed optoelectronic transmitter is realized by means of two separate chips being hybrid mounted and electrically interconnected such that any parasitic influences are minimized. A first integrated semiconductor chip may contain the electrical transmission line and the drive circuits while a second integrated semiconductor chip may contain the optical source signal generator and the continuous optical waveguide.

A chip of the latter kind can, in the TWEAM-case, be based directly on a single-section TWEAM design with integrated termination resistors. However, the metal electrode on top of the mesa should be discontinued for a short distance (in the order of 10 μm) to provide a resistance between adjacent modulator sections, which is substantially larger than the termination resistance for each section. Thereby the risk of any backward waves propagating along the modulator is avoided. Nevertheless, the epitaxial layer may continue along the entire length of the TWEAM modulator structure. The processing of a multi-section TWEAM-chip thus becomes identical to that of a single-section TWEAM-chip. It is advantageous if the signal and ground pads of the two chips are mounted edge to edge with a minimum distance, such that the bonding parasitics are reduced.

According to an alternative embodiment of the invention, units are integrated onto a single semiconductor chip.

Below follows relevant numerical comparative examples with reference to TWEAM technology in order to demonstrate the usefulness of the suggested modulator structure. The bitrates have been chosen to 10 Gb/s, 25 Gb/s, 40 Gb/s, 50 Gb/s, 100 Gb/s and 160 Gb/s respectively. The indicated bandwidth relates to one modulator section of the length 1 .0 mm, 0.6 mm, 0.4 mm, 0.3 mm, 0.15 mm and 0.08 mm respectively. The bandwidths are chosen to comply with (some margin) to the corresponding bit- rate. The indicated voltages represent a single-ended swing across one drive circuit output. The values for the single-section modulators have actually been measured with respect to the lengths 0.25 mm, 0.45 mm and 0.95 mm, whereas the values for the lengths 0.08 mm and 0.15 mm have been derived by means of extrapolation. The ER values have been assumed to be 3dB or better for datacommunications (Ethernet) at 10 Gb/s and 4x25 = 2x50 = 100 Gb/s, and 10 dB for telecommunications at 10 Gb/s, 40 Gb/s and 160 Gb/s.

^*) No substantial benefit Naturally, multiple-section modulators require specially designed drive circuits having the same number of outputs as there are modulator sections. This might complicate the solution to some extent. However, it is doubtless worth the effort, since for any given speed the ER is improved at least by a factor two (see the table above). It should also be noted that it is preferable to fabricate the connections between the drive circuit outputs and the modulator sections with very low series inductances for high speeds. This is especially important if the individual modulator sections have relatively low impedances, say 20 - 30 ohms. Due to the fact that the drive circuits are fed via an electrical transmission line having delays that match the respective delays along the modulator, problems related to losses may occur at high speed when transistor technologies with a conducting sub- strate, such as silicon, are used.

In order to sum up, the general method according to the invention will now be described with reference to a flow diagram in figure 6.

Two parallel steps 601 and 602 receive a primary electrical signal S_e and a continuous wave optical signal CW₀ respectively. A step 603 following the step 601 amplifies the primary electrical signal S_e into a first amplified signal S'_e. A step 604 modulates the continuous wave optical signal CW₀ received in the step 602 with the first amplified signal S'_e and thereby produces a first modulated optical signal s'₀. A subsequent step 605 produces a first delayed electrical signal S_e ^d1 by delaying the primary electrical signal S_e, such that an amplified version of the first delayed electrical signal S_e ^d1 is aligned in phase with the first modulated optical signal s'₀. After that, a step 606 effectuates the amplification of the first delayed electrical signal S_e ^d1 into a second amplified signal S"_e. A following step 607, then modulates an aggregated signal of the continuous wave optical signal CW₀ and the first modulated optical signal s'₀ with the second amplified signal S"_e. Thereby a second modulated optical signal s"₀ is produced (analogous to the step 604 above).

A subsequent step 608 "checks" whether further modulation stages should be applied, and if so, returns the procedure directly to the steps 601 and 602. Otherwise, the procedure continues to a step 609 where the first modulated optical signal s'_o and the second modulated optical signal s"₀ are aggregated into a resulting optical signal S₀ to be fed out. If the procedure loops through the steps 601 - 607 one or more additional times, at least a third modulated optical signal s'"₀ is also included in the resulting optical signal S₀. As mentioned earlier, an arbitrary number n of such loops may be performed according to the invention.

Although the flow diagram in figure 6 refers to a sequential procedure of steps, in practice, all the steps are actually carried out simultaneously, however with respect to different infinitesi- mally small signal units. The described sequential procedure is namely true only for each such signal unit.

The term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

1 . An optoelectronic transmitter for receiving a primary electrical signal (S_e) and producing in response thereto a resulting optical signal (S₀), which contains the same information as the primary electrical signal (S_e), the transmitter comprising: an optical source signal generator (410) producing a continuous wave optical signal (CW₀), a first drive circuit (423a) receiving the primary electrical signal (S_e) and producing in response thereto a first amplified electrical signal (S'_e), characterized in that it comprises: an electrical transmission line (424) receiving the primary electrical signal (S_e), at least one second drive circuit (423b, 423c) receiving a delayed version of the primary electrical signal (S_e ^d1 ; S_e ^d2) via the electrical transmission line (424; 424a) and producing in response to this signal (S_e ^d1 ; S_e ^d2) at least one second amplified electrical signal (S"_e, S'"_e), and a continuous optical waveguide (421 ) receiving the continuous wave optical signal (CW₀), receiving the first amplified electrical signal (S'_e), receiving the at least one second amplified electrical signal (S"_e, S'"_e) and producing in response to the received signals (CW₀, S'_e, S"_e, S'"_e) the resulting optical signal (S₀), the at least one second amplified electrical signal (S"_e, S'"_e) entering the continuous optical waveguide (421 ) such that it is aligned in phase with an already modulated optical signal (s'₀) therein resulting from the first amplified electrical signal (S'_e).

2. An optoelectronic transmitter according to claim 1 , characterized in that the continuous optical waveguide (421 ) includes one modulator section (421 a-421 c) for each of the drive circuits (423a-423c), each modulator section (421 a-421 c) receiving a respective amplified electrical signal (S'_e, S"_e,

S"e), receiving at least one optical signal (CW₀, s'₀, s"₀), and in response thereto producing a modulated optical signal (s'_o, s"₀, S"O), the resulting optical signal (S₀) representing an aggregation of these modulated optical signals (s'₀, s"₀, s'"₀).

3. An optoelectronic transmitter according to any one of the claims 1 or 2, characterized in that a first end of the electrical transmission line (424) receives the primary electrical signal (S_e) and a termination resistor (425) in a second end of the electrical transmission line (424) terminates any remaining energy (S_e ^d3) in the primary electrical signal (S_e).

4. An optoelectronic transmitter according to any one of the claims 2 - 3, characterized in that the electrical transmission line

(424) includes a delay element (424a, 424b) between the first drive circuit (423a) and each of the at least one second drive circuit (423b, 423c), each delay element (424a, 424b) imposing a delay on the electrical signal (S_e) being equivalent to a delay that each of the modulator sections (421a - 421 c) imposes on the modulated optical signal (s'₀, s"₀, s'"₀).

5. An optoelectronic transmitter according to any one of the preceding claims, characterized in that the electrical transmission line (424), the first drive (423a) and the at least one second drive circuit (423b, 423c) are integrated onto a first semiconductor chip, and the optical source signal generator (410) and the continuous optical waveguide (421 ) are integrated onto a second semiconductor chip.

6. An optoelectronic transmitter according to any one of the claims 1 - 4, characterized in that the electrical transmission line (424), the first drive (423a), the at least one second drive circuit (423b, 423c), the optical source signal generator (410) and the continuous optical waveguide (421 ) are all integrated onto a single semiconductor chip.

7. An optoelectronic transmitter according to any one of the claims 2 - 6, characterized in that the modulator sections (421 a

- 421 c) are traveling wave electroabsorption modulators.

8. An optoelectronic transmitter according to claim 3, characterized in that the modulator sections (421 a - 421 c) are lumped electroabsorption modulators.

9. An optoelectronic transmitter according to any one of the claims 2 - 6, characterized in that the modulator sections (421 a

- 421 c) are Mach-Zender modulators.

10. An optoelectronic transmitter according to any one of the preceding claims, characterized in that the resulting optical signal (S₀) has a non-return-to-zero signal format.

1 1 . A method of converting an electrical signal into an optical signal, which contains the same information as the electrical signal, comprising the steps of: receiving a primary electrical signal (S_e), receiving a continuous wave optical signal (CW₀), amplifying the primary electrical signal (S_e) into a first amplified electrical signal (S'_e), modulating the continuous wave optical signal (CW₀) with respect to the first amplified electrical signal (S'_e) into a first modulated optical signal (s'₀), characterized by the steps of: producing a first delayed electrical signal (S_e ^d1 ) by delaying the primary electrical signal (S_e) in proportion to a processing delay in producing the first modulated optical signal (s'_o) such that an amplified version (S"_e) of the first delayed electrical signal (S_e ^d1) is aligned in phase with the first modulated optical signal (s'₀), amplifying the first delayed electrical signal (S_e ^d1) into a second amplified electrical signal (S"_e), modulating at least the first modulated optical signal (s'₀) with respect to the second amplified electrical signal (S"_e) into a second modulated optical signal (s"₀), and forming a resulting optical signal (S₀) by aggregating at least the first modulated optical signal (s'₀) and the second modulated optical signal (s"₀).

12. A method according to claim 1 1 , characterized by the further steps of: producing at least one second delayed electrical signal (S_e ^d2) by delaying the primary electrical signal (S_e) in proportion to a processing delay in producing the second modulated optical signal (s"₀) such that an amplified version (S'"_e) of the second delayed electrical signal (S_e ^d2) is aligned in phase with the second modulated optical signal (s"₀), amplifying the second delayed electrical signal (S_e ^d2) into a third amplified electrical signal (S'"_e), modulating at least the second modulated optical signal (s"₀) with respect to the third amplified electrical signal (S'"_e) into a third modulated optical signal (s'"₀), and forming the resulting optical signal (S₀) by aggregating at least the first modulated optical signal (s'₀), the second modulated optical signal (s"₀) and the third modulated optical signal

13. A method according to claim 12, characterized by repeating the delaying, amplifying modulating and aggregating steps with respect to at least one additional cycle.

14. A method according to any one of the claims 1 1 - 13, characterized by the modulating steps involving traveling wave electroabsorption modulation.

15. A method according to any one of the claims 1 1 - 13, characterized by the modulating steps involving concentrated electroabsorption modulation.

16. A method according to any one of the claims 1 1 - 13, characterized by the modulating steps involving Mach-Zender modulation.

17. A method according to any one of the claims 1 1 - 16, characterized by the resulting optical signal (S₀) having a nonreturn-to-zero format.