JP3527617B2 - Standard optical frequency generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a standard optical frequency generator for generating a standard optical frequency in a wide band using a semiconductor laser device. This wideband standard optical frequency is used, for example, as a reference light source of a wavelength division multiplexing transmission system.

[0002]

2. Description of the Related Art As a technique for generating a standard optical frequency in a wide band, a method of utilizing an optical modulation sideband generated by modulating continuous monochromatic light (CW light), which is a reference, has been proposed.

FIG. 6 shows the configuration of a conventional standard optical frequency generator using an optical modulation sideband. In the figure,
The CW light source 61 generates reference CW light having an oscillation frequency ν according to the reference light frequency. The optical modulator 62 uses this reference CW.
The microwave and the modulation signal of the microwave output from the microwave oscillator 63 of low phase noise are input. In the optical modulator 62,
Generally, a symmetric Mach-Zehnder interferometer type optical modulator with a high extinction ratio is used, but this is a resonator for microwaves by integrally forming a phase-matched optical waveguide and a microwave waveguide on a lithium niobate crystal. It is configured as follows. Microwaves are enhanced in the resonator, enabling low-power, large-amplitude optical modulation. The microwave oscillator 63 is a standard commercial product that can generate high-quality microwaves with single sideband phase noise suppressed to 100 dBc / Hz or less over the entire band over a wide band (up to 50 GHz). Used.

By the optical modulation with such a configuration, an optical modulation side band is generated for each modulation frequency centering on the input reference light. When this optical modulation sideband is generated by a Mach-Zehnder intensity modulator based on phase modulation,
The optical frequency spectrum presents a pyramid-shaped comb with the input light as the apex, and the intensity of each component is that described by the Bessel function.

Here, a wideband standard optical frequency can be generated by arranging optical modulation sidebands generated from a plurality of CW lights so that their tails coincide with each other, as shown in FIG. . Phase-locked loop stabilization technology is used to match the tails. this is,
One mode is extracted from each of the overlapping skirts of the two sidebands, and the oscillation frequency of the input light of one sideband is controlled so that the phases of the two modes are synchronized.

FIG. 8 shows an example of the configuration of a conventional standard optical frequency generator using a phase locked loop. In the figure, an optical modulation sideband 1 is generated using a CW light source 61-1 having an oscillation frequency ν ₀ , an optical modulator 62-1 and a microwave oscillator 63-1 having an oscillation frequency ΔF. The mode of (ν ₀ + NΔF) from this optical modulation sideband 1 is set to the optical splitter 64.
-1 and the band pass filter 65-1. On the other hand, a CW light source 61-2 having an oscillation frequency ν, an optical modulator 62-2, and a microwave oscillator 63-2 having an oscillation frequency ΔF.
Is used to generate the light modulation sideband 2. The mode of (ν-MΔF) is extracted from the optical modulation sideband 2 via the optical splitter 64-2 and the bandpass filter 65-2 (N and M are arbitrary integers, N = N in the example of FIG. 7).
M = 5).

In the (ν-MΔF) mode, a frequency difference is provided by the acousto-optical frequency shifter 66-1 by the frequency f of the electric oscillator 67-1. The (ν-MΔF + f) mode and the (ν ₀ + NΔF) mode are combined by the multiplexer 68-1 and input to the photodetector 69-1 to be converted into an electric signal. This electric signal and the signal of frequency f are mixed by the mixer 70-
1, the relative phase error between the two modes is detected, and the CW light source 61-2 is negatively feedback-controlled by using the detected phase error as the error signal, whereby the phase synchronization is realized.

Similarly, by controlling the oscillation frequency of the input light of one sideband so that the phases of the two modes are sequentially synchronized, a wideband standard optical frequency can be obtained.

[0009]

In the conventional standard optical frequency generator shown in FIG. 8, the number of control circuits forming a phase locked loop increases in proportion to the number of optical combs. Moreover, each control circuit is complicated as shown in FIG. 8, and in a configuration that requires many CW light sources to obtain a standard optical frequency in a wide band,
It is more complicated and extremely expensive.

Further, in the series connection system of the phase locked loop,
There was a problem that the system became unstable due to the accumulated error. By the way, there is an optical injection locking technology as an alternative to the phase locked loop stabilization technology. However, in the configuration in which a new light modulation sideband is formed around the wavelength of the mode extracted from the skirt of the light modulation sideband, half of the light modulation sidebands completely overlap each other, resulting in extremely poor efficiency, It is not suitable for obtaining a wide band standard optical frequency.

An object of the present invention is to provide a standard optical frequency generator capable of generating a standard optical frequency whose absolute wavelength and frequency interval are simultaneously controlled in a wide band with a simple structure.

[0012]

In the standard optical frequency generator of the present invention, a plurality of mode-locked semiconductor lasers having different gain bands are cascaded so that the tails of optical spectra overlap each other. The configuration is such that the absolute wavelength and the pulse phase (timing) match.

Generally, the absolute wavelength and the pulse phase (timing) of the mode-locked semiconductor laser are controlled simultaneously by injecting an optical signal whose phases are synchronized with each other and whose absolute wavelength is controlled into the mode-locked semiconductor laser. (Reference: Z. Ahmed, et al., "Locking characterist
ics of a passively mode-locked monolithic DBR lase
r stabilized by optical injection ", IEEE Photonics
Technol. Lett., 8 (1), pp.37-39, 1996).

Therefore, a plurality of mode-locked semiconductor lasers having different gain bands are prepared so that the tails of the optical spectra are overlapped with each other as shown in FIG. 2. First, for example, the mode-locked semiconductor laser on the shortest wavelength side is used as the reference CW light. The absolute wavelength and pulse phase (timing) are controlled by injection and forced or phase locked loop stabilization. Furthermore, by branching a part of the output pulse and injecting it sequentially into adjacent mode-locked semiconductor lasers, the absolute wavelength and pulse phase (timing) of all mode-locked semiconductor lasers can be synchronized, and as a result, It is possible to generate a standard optical frequency in a wide band with a stable absolute wavelength and frequency interval.

The mode-locking semiconductor laser into which the reference CW light is first injected is not limited to the one having the shortest wavelength side, and for example, the mode-locking semiconductor laser having the longest wavelength side may be sequentially controlled to the short wavelength side. .

[0016]

1 shows the basic configuration of a standard optical frequency generator of the present invention. As shown in FIG. 2, the plurality of mode-locked semiconductor lasers 11-1, 11-2, and 11-3 have different gain bands so that the tails of the optical spectra overlap each other. Among them, the reference CW light source 12 inputs the reference CW light of the optical frequency ν ₀ to the mode-locking semiconductor laser 11-1 having the shortest wavelength gain, and the microwave oscillator 13 inputs the modulation signal of the frequency ΔF. , Synchronize to the reference wavelength and reference timing by forcing or stabilizing the phase-locked loop. A part of the output pulse 1 of the mode-locked semiconductor laser 11-1 is branched via the optical splitter 14-1 and injected into the adjacent mode-locked semiconductor laser 11-2 for synchronization. Thereby, the mode-locked semiconductor laser 11-2
Output pulse 2 absolute wavelength and pulse phase (timing)
Can be synchronized.

Next, a part of the output pulse 2 of the mode-locked semiconductor laser 11-2 is branched via the optical splitter 14-2, and light is injected into the adjacent mode-locked semiconductor laser 11-3 for synchronization. Hereinafter, by sequentially performing such branching and injection, it is possible to control so that the absolute wavelengths and pulse phases (timings) of all mode-locked semiconductor lasers match. Thereby, a wideband standard optical frequency generator can be realized.

Hereinafter, a concrete configuration example of each unit will be described. As the reference CW light source 12, a narrow line width DFB laser whose optical frequency is locked to the absolute wavelength of 193.1 THz based on the absorption line of HCN gas can be used.

As the mode-locked semiconductor laser 11, the conventional multi-electrode integrated mode-locked semiconductor laser shown in FIG. 3 can be used. The pulse phase (timing) is controlled by applying a modulation signal from the outside to the segment 1. At present, a repetition frequency of about 25 GHz can be stably realized. The element length is about 1.7 mm when the refractive index is 3.5.
Is the optimum value. The synchronization with the reference CW light is segment 2
The current is controlled by injecting current into the phase adjustment region. In addition,
Segment 3 is the DBR region and segment 4 is the gain region. The electrodes are divided by shallow etching that does not reach the active layer, and a separation resistance of 1 kΩ is realized while suppressing internal reflection as much as possible.

Further, the mode-locked semiconductor laser 11 has
The harmonic collision pulse mode-locked semiconductor laser shown in FIG. 4 can be used. Here, FIG. 4A shows the structure of a conventional collision pulse mode-locked semiconductor laser in which a gain region 21 is arranged at both ends and a saturable absorption region 22 is arranged in the center. This is the basic unit of the harmonic collision pulse mode-locked semiconductor laser. FIG. 4 (b) shows the structure of a harmonic collision pulse mode-locked semiconductor laser constructed by arranging the basic units in series.

In the collision pulse mode-locked semiconductor laser (basic unit) shown in FIG. 4 (a), since two pulses circulate in the laser resonator, a pulse train having a repetition frequency twice the fundamental mode interval of the laser resonator. Can be generated. When the length of the basic unit is Lo, the effective refractive index is n, and the speed of light in vacuum is c, the mode interval is ΔFm = c / (nLo). For example, if n = 3.5 and Lo = 450 μm, the repetition frequency is 190 GHz.

In the structure of the harmonic collision pulse mode-locked semiconductor laser shown in FIG. 4B, the repetition frequency of the harmonic mode-locked pulse is basically unchanged if the basic units are the same. In this case, assuming that the number of basic units is N, the number of circulating pulses is 2N, and the basic mode interval is reduced to 1 / N. However, since the length of the gain region is N times, it is possible to expect a great improvement in strength compared to the basic unit.

However, as N increases, the fundamental mode interval decreases, and harmonic mode locking may become unstable. However, as shown in FIG. 4B, the circulating pulses collide with each other in the saturable absorption region 22 at the same time, so that only one harmonic mode-locked state is stabilized.

The harmonic collision pulse mode-locked semiconductor laser can be realized by a multi-electrode structure by electrode separation by etching, like the conventional multi-electrode integrated mode-locked semiconductor laser.

A mechanism for stabilizing the harmonic collision pulse mode-locked semiconductor laser by light injection will be described with reference to FIG. The first harmonic collision pulse mode-locked semiconductor laser outputs an optical pulse (super mode original) whose absolute wavelength and pulse phase (timing) are stabilized by injection of the reference CW light, and outputs the second harmonic collision pulse. It is injected into a mode-locked semiconductor laser. here,
The super mode is a mode locked by harmonic mode synchronization, and a sixth-order example is shown in the figure.

If the second harmonic impinging pulse mode-locked semiconductor laser exhibits supermode 1 which matches the original supermode, the supermode is locked to the original by light injection. On the other hand, even in the case of the super mode 2 that does not match the original super mode, the suppressed fundamental cavity mode and the original are in harmony, so the super mode 2 at the time of non-injection is suppressed by the light injection and the super mode 3 in harmony grows. To do.
That is, even if the super modes do not match, the optimum super mode is automatically selected by light injection.

As described above, even when the repetition frequency is high and the oscillation frequency cannot be easily adjusted by the injection current or the like, the suppressed cavity mode has a high significance, and the absolute wavelength and the pulse phase ( timing)
Can be stabilized.

As a method of stabilizing the pulse phase (timing) of such a harmonic collision pulse mode-locked semiconductor laser, a phase-locked loop stabilizing circuit for a passive mode-locked semiconductor laser, which has been conventionally proposed,
A frequency division hybrid mode locking method or the like can be used.

[0029]

As described above, the standard optical frequency generator of the present invention can generate a standard optical frequency in a wide band in which the absolute wavelength and the frequency interval are simultaneously controlled with an extremely simple and inexpensive structure.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of a standard optical frequency generator of the present invention.

FIG. 2 is a diagram showing a relationship between gain bands of a plurality of mode-locked semiconductor lasers.

FIG. 3 is a diagram showing the structure of a multi-electrode integrated mode-locking semiconductor laser.

FIG. 4 is a diagram showing a structure of a harmonic collision pulse mode-locked semiconductor laser.

FIG. 5 is a diagram illustrating a mechanism for stabilizing a harmonic collision pulse mode-locked semiconductor laser by light injection.

FIG. 6 is a diagram showing a configuration of a conventional standard optical frequency generator using an optical modulation sideband.

FIG. 7 is a diagram showing that two sidebands can be synchronized by phase-locking at a hem portion.

FIG. 8 is a diagram showing a configuration example of a conventional standard optical frequency generator using a phase locked loop.

[Explanation of symbols]

11 Mode-locked semiconductor laser 12 Standard CW light source 13 Microwave oscillator 14 Optical splitter 21 Gain area 22 Saturable absorption region

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Claims (2)

(57) [Claims] 1. A plurality of mode-locked semiconductor lasers having different gain bands so that the tails of optical spectra are overlapped with each other, a reference CW light source for outputting reference CW light of a predetermined optical frequency, and a modulation signal of a predetermined frequency. And a plurality of optical branching means for branching a part of the optical pulse output from each of the mode-locking semiconductor lasers, wherein any one of the plurality of mode-locking semiconductor lasers is used as a first mode. In the case of a synchronous semiconductor laser, the first mode-locking semiconductor laser inputs the reference CW light and the modulation signal, outputs a first optical pulse having a stabilized absolute wavelength and phase, and outputs the first optical pulse. Each mode-locking semiconductor laser subsequent to the second mode-locking semiconductor laser adjacent to the mode-locking semiconductor laser is an optical pulse output from the preceding mode-locking semiconductor laser. It said light branching means to the light injected via a standard optical frequency generator, characterized in that sequentially outputting light pulses synchronized with the absolute wavelength and the phase of the previous mode-locked semiconductor laser. 2. A mode-locked semiconductor laser comprises a collision pulse mode-locked semiconductor laser having a gain region at both ends thereof and a saturable absorption region at the center thereof as a basic unit, and the basic units are arranged in series. The standard optical frequency generator according to claim 1, wherein the standard optical frequency generator is a harmonic collision pulse mode-locked semiconductor laser.