DE102010011615A1 - Optical pulse widening device for white light source, utilized for calibrating optical sensor, has coupling device coupling laser beams into light conductors, and uncoupling device streamlines together light beams from conductors - Google Patents

Abstract

The device (10) has a set of light conductors (12.1, 12.2, 12.3) i.e. single-mode fibers, provided with different optical lengths, where a light guard length linear distribution is provided along the optical lengths of the conductors. A coupling device (14) couples laser beams (20) into the conductors. An uncoupling device (16) streamlines the light beams together from the conductors, where the conductor are formed such that a maximum delay difference between the two conductors amounts to 0.8 times of an inverse repetition rate. Independent claims are also included for the following: (1) a light source comprising a pulsed laser for delivering light with a repetition rate and a pulse duration (2) a method for generating light (3) a method for generating a light curve with a predetermined time changing frequency or wavelength (4) a method for calibrating an optical sensor (5) a method for manufacturing a pulse form device for forming a light pulse.

Description

The invention relates to an optical pulse broadening device. According to a second aspect, the invention relates to a method for generating light, a method for generating a light path and a method for calibrating an optical sensor.

For the spectral calibration of optical receivers such as solar cells, UV radiometers, photometers, filter receivers and spectroradiometers, monochromators are currently being used which separate a narrow wavelength range from the spectrum of a white light source with the aid of optical gratings or prisms. However, this gives only low radiation powers, which are not sufficient for some applications.

It is therefore desirable to be able to use high-performance, spectrally tunable lasers.

On the one hand, there are real, continuously operating lasers (cw lasers, English: continuous wave lasers, lasers with constant radiation intensity), but these have the disadvantage that they cause interference problems due to their high coherence length.

It is a further disadvantage that these cw-lasers are not automatically tunable over the entire required wavelength range.

On the other hand, there are pulsed lasers with repetition rates of at present 80 MHz, which are automatically tunable over the entire required wavelength range. Such pulsed lasers emit short pulses at a repetition rate. For example, such pulsed lasers emit pulses less than 200 fs every 12 ns. This light is not suitable, for example, for the calibration of optical receivers, since supersaturations with minority carriers occur in the receivers. The problem is particularly relevant in the blue and UV spectral regions, where all radiation is absorbed in a small, near-surface region of the receiver.

The invention has for its object to make tunable laser light with a temporally uniform as possible light intensity course available.

The invention solves the problem by an optical pulse broadening device with (i) a plurality of optical fibers of different optical lengths, (ii) a coupling device for coupling a laser beam into the optical fibers and (iii) a coupling device for combining light beams from the optical fibers.

According to a second aspect, the invention solves the problem by a method of generating light comprising the steps of (a) generating pulsed laser light having a repetition rate and a pulse duration, (b) coupling the laser light into an optical pulse broadening device, which differs a plurality of optical fibers optical lengths, so that a plurality of time-shifted partial light pulses from each light pulse of the pulsed laser light is formed, and (c) combining at least a portion of the partial light pulses to a smoothed output light signal.

An advantage of the invention is that so light of a desired wavelength of a pulsed laser can be produced, which is smoothed. That is, light which is substantially the same as that of light from a cw laser and yet has the tunability of a pulsed laser can be obtained. It is additionally advantageous here that an intensity profile of the light deviates significantly less from an average over time in the quadratic mean than in the case of a pulsed laser.

Ulbrichtkugeln have been used in the prior art for stretching laser pulses. Compared to an integrating sphere, the invention results in no fluorescence being produced. This is particularly advantageous when blue or UV light is needed. Instead of pulse broadening, it is also possible to speak of a pulse multiplication, a pulse smoothing or a pulse shaping.

It is also advantageous that the optical pulse broadening device has only a low absorption. In this way, particularly intense light can be produced. This is particularly advantageous when the optical pulse broadening device is part of a device which already has a waveguide for guiding the light. Due to the low absorption, the output signal over time can be regarded as quasi-constant and generates, in contrast to the use of an integrating sphere, no significant exponential decrease.

It is a further advantage that the peak power of the emergent light is significantly smaller than that of the light of the light source, without significantly changing the average power. This is beneficial in protecting the eyes and reducing the damage threshold in material, tissue or fluorescence studies.

In the context of the present description, a light guide is understood in particular to be a glass fiber. In particular, each device is understood by the coupling device, by means of which an incident in the coupling device Laser beam is distributed to the light guide. It is particularly favorable if the coupling-in device is designed such that essentially the same light intensity is dispensed with on each light guide. By the feature that substantially the same light intensity is coupled into the light guides, it is understood in particular that it is desirable that exactly the same light intensity be coupled in each of the light guides, but that this is not necessary. For example, it is tolerable that the maximum relative difference in light intensities between two light guides is less than 25%.

If the intensity distribution (more correctly, albeit less common: irradiance distribution) at the input of the pulse widening device is known, then this can be taken into account by the length distribution of the individual fibers by adjusting the distance difference between this and the next fiber. This means that if the brushing strength is lower than the mean, the next longer fiber must have a smaller length increase than on average. The increase in length must ideally be proportional to the power contribution at the exit of the fiber. This power is in turn proportional to the product of irradiance and transmission of the fiber. In other words, the ratio of the path difference between the ith and i + 1th fibers and the power given by the intensity and angle distribution at the input of the ith fiber and the thickness and transmission of the i th fiber at the exit of this i-th fiber is preferably constant.

Alternatively, it is possible to randomly distribute the fibers at the entrance. So it is ensured that z. As the edge fibers, where typically the irradiance or intensity is the lowest and the angular distribution most unfavorable, not all almost equal lengths, but that there are some short, some middle and some longer individual fibers attached.

The coupling-out device is understood in particular to be any device which is designed to align partial laser beams emerging from the light guides so that they can be coupled into a subsequently arranged device.

The decoupling device and the coupling device are preferably designed so that they merge the optical fibers at one point. It can be provided, for example, that the coupling device and / or the decoupling device is equipped via a coupling device for connection to other optical fibers for supplying or discharging laser light or free jet or coupling into a monochromator.

It is advantageous if the optical fibers are made of the same material and have different optical fiber lengths. So the light guides can be easily made.

The optical length is the product of the refractive index and the length of an object. The use of the same material for all optical fibers leads to a particularly easy to manufacture pulse broadening device.

According to a preferred embodiment, the optical fiber lengths have an optical fiber length distribution, wherein the optical fiber length distribution has a square sum in the form of the sum of the squares of the deviations from a uniform distribution and wherein the square of the sum of squares divided by the number of optical fibers is at most 0.5, in particular 0.1. The optical fiber length distribution is the assignment of a serial number n of the optical length sorted optical fiber to the optical length. Ideally, the optical lengths are linearly distributed, that is, the difference in the optical lengths of two optical fibers of the sorted optical fiber is constant.

Such a linear distribution is desirable, but not necessary. It is thus possible to lay a compensation straight line through the optical lengths, in particular the lengths of optical fibers, and to sum up the quadratic deviations from this linear interpolation. The root of the sum of squares divided by the number of optical fibers is a measure of the deviation from an ideal linear distribution and should therefore be as small as possible. It is particularly favorable if this value is less than 0.1 or 0.01.

In other words, when the optical fibers are sorted according to their optical lengths, the difference between two adjacent optical fibers is ideally an equal distribution so that all the differences are equal. However, small deviations from an average of all differences of, for example, 25% are possible.

According to the invention, a light source is also provided with (a) a pulsed laser which is designed to emit light at a repetition rate and a pulse duration, and (b) a pulse width distribution device according to the invention, which is arranged so that the light can be coupled into the pulse widening device and thereby one Pulse width distribution of the light is achievable. Such a light source has an output signal to light whose intensity distribution is smoothed. For this reason, the light source is particularly well suited for the calibration of optical sensors.

The light source preferably has a coupling-out device, which is designed to couple light leaving the pulse widening device into an optical waveguide. It is particularly favorable if the light source also comprises an optical fiber, for example a single-mode fiber, by means of the light generated by the light source. It is also possible a free jet or a decoupling in a monochromator to narrow the light and thus to widen the individual pulses.

Optionally, the laser is tunable, in an interval of 2000 nm to 230 nm. It is particularly favorable if the laser is automatically tunable, that is, the wavelength of a control preprogrammed time-dependent controlled or can be controlled. Such a light source emits a smoothed light of a preselectable wavelength in the optical or adjacent UV range, which is particularly advantageous for the calibration of optical sensors, for example.

Preferably, the coupling device is designed so that substantially the same light intensity is coupled into each light guide. By this is to be understood that it is desirable that in each light guide the same intensity is coupled, but this is not necessary. Thus deviations are tolerable. For example, the light intensity coupled into an optical fiber may differ by 25% from an average across all optical fibers.

The maximum transit time difference between the shortest and the longest optical fiber should be at least 0.5 times the inverse repetition rate. The mean transit time difference between the ith and the i + 1th fibers should be T_Rep / N to obtain an equal distribution, where T_Rep is the repetition time and N is the number of fibers. Preferably, the light guide is designed so that a maximum transit time difference between two light guides, usually the shortest and the longest light guide, is at least 0.8 times an inverse repetition rate.

In the following, the invention will be explained in more detail with reference to an exemplary embodiment. It shows

1 an optical pulse broadening device according to the invention in a light source according to the invention;

2 a diagram showing the light intensity as a function of time for a laser and a light source according to the invention; and

3 a graphical representation of an optical fiber length distribution of the optical fibers in a pulse broadening device according to 1 ,

1 shows a pulse broadening device 10 containing a variety of fiber optics 12 namely, the light guides 12.1 . 12.2 . 12.3 , ... includes. The pulse broadening device 10 also has a coupling device 14 and a decoupling device 16 on.

The light guides 12 differ in their optical fiber lengths L. Thus, the optical fiber length L3 of the third optical fiber 12.3 greater than the optical fiber length L2 of the optical fiber 12.2 and these in turn larger than the optical fiber length L1 of the optical fiber 12.1 , The light guides 12 are all made of the same material, namely glass, so that the optical lengths L _{opt are} each proportional to the optical fiber length L and also for all optical fibers 12 differ.

1 also shows a schematically drawn pulsed laser 18 which is adapted to emit light 20 with a repetition rate f _rep and a pulse duration T. For example, the repetition rate is above 50 MHz. The pulse duration can be, for example, between 100 and 500 fs.

The laser 18 is tunable and is in the lag, light 20 in the wavelength range at least in the range of interest sensitivity of the sensor. The light 20 is through the coupling device 14 in the light guides 12 coupled, so that in each of the light guides a light intensity I is applied.

The coupling device 14 is designed so that the light intensities in the individual light guides 12 differ only slightly. For example, the intensity I1 in the light guide 12.1 essentially equal to the intensity I2 in the light guide 12.2 , The intensity I in one of the light guides can differ from a medium intensity I _mean , which is due to averaging over all light guides 12 is obtained, for example, differ by 25%. In every light guide 12 this creates a partial light pulse. The partial light pulses are from the decoupling device 16 combined into a smoothed output light pulse.

2 shows a diagram in which on the one hand the intensity I ₁₄ , which is the light 20 when entering the coupling device 14 has, and on the other hand, an intensity I _{16 (t)} , which is an output light beam 22 after leaving the decoupling device 16 has, is applied. The intensity I, which can be given for example by the laser power, is normalized to the peak power as 1.

It can be seen that from the laser 18 originating output signal in the form of light 20 has a pulse duration T and a repetition rate f _rep that corresponds to a repetition time T _rep . In 2 the time t is normalized to the repetition time Trep. Between two pulses, the intensity I is in good approximation 0. The light intensity of the output light beam 22 (see. 1 ) fluctuates around a mean M.

3 shows a distribution of the optical fiber lengths L and whose deviation is based on a linear distribution. The X-axis is a serial number n of the light guides arranged according to the length of the light guide. The larger n, the larger the optical length L _opt or in the present case, the optical fiber length L.

On the ordinate, the light guide length L is plotted for the respective light guide.

It can be seen that the optical fiber lengths L of the optical fibers have an approximately linear distribution, which can be described by a straight line g. For each serial number n, a deviation d from the linear distribution can be calculated, as shown for the serial number n = 8. The sum Q = Σnd ² (n) The sum of the squares with the deviation from the linear distribution therefore provides a measure of how exactly the actual distribution of the optical fiber lengths L corresponds to the ideal of the linear distribution.

The root of the sum of squares divided by the number of light guides N is called σ, so it holds

The smaller σ is, the better the actual distribution of the optical fiber lengths L corresponds to the ideal linear distribution.

Each of the light guides 12 conditionally compared to another light guide a propagation delay. That is, light is a longer time from the coupling device 14 to the coupling-out device 16 (see. 1 ), the longer the respective light guide 12 is. The difference in the transit times of the partial light pulses in two light guides is the transit time difference Δτ. τ _n denotes the time it takes for a light pulse to pass through the n-th light guide 12.n to run. There is a maximum transit time difference Δτ _max between the shortest optical fiber and the longest optical fiber. It therefore applies to all N light guides 12.1 , ..., 12.N

A particularly favorable case arises when the maximum transit time difference Δτ _max corresponds approximately to the inverse repetition rate, ie T _rep . In this case, the pulsed laser light is smoothed particularly efficiently. For example, the maximum transit time difference between two optical fibers is at least 0.8 times an inverse repetition rate T _rep .

• (a) Bestimmen einer Stammfunktion F(t) in den Intervallgrenzen mit der Randbedingung, dass F(0) = 0 ist,
• (b) Normieren der Stammfunktion F(t), so dass F(T_Rep) = N ist, wobei N die Anzahl der Einzelfasern in dem Bündel ist,
• (c) aus der Umkehrfunktion t_i = F^ – 1(i) mit i = 1, ..., N ermitteln der Verzögerungszeiten, die mit den einzelnen Fasern erzielt werden müssen,
• (d) Zuordnen einer Faserlänge l_i zu jeder Verzögerungszeit t_i nach der Formel l_i = c·t_i/n, wobei c die Lichtgeschwindigkeit und n die optische Brechtahl des Lichtleiters ist.

According to the invention, a pulse shaping device is also provided for shaping a light pulse so that it has a predetermined signal curve f (t) at an interval between two light pulses, wherein the light pulses have a time interval of a repetition time T_rep, with a number N of individual optical fibers having a length distribution l (i) assigning the length l to each optical fiber index i, the length distribution being determinable by the following method:

• (a) determining a parent function F (t) in the interval boundaries with the constraint that F (0) = 0,
• (b) normalizing the parent function F (t) such that F (T_Rep) = N, where N is the number of individual fibers in the bundle,
• (c) from the inverse function t_i = F ^ - 1 (i) with i = 1, ..., N determine the delay times that have to be achieved with the individual fibers,
• (d) assigning a fiber length l_i at each delay time t_i according to the formula l_i = c · t_i / n, where c is the speed of light and n is the optical refractive index of the optical fiber.

By an additional extension of the entire optical fiber bundle, a further smoothing of the output light can be achieved.

LIST OF REFERENCE NUMBERS

10

Pulse broadening device

12

optical fiber

14

coupling device

16

decoupling

18

laser

20

light

22

Output light beam

L

Optical fiber length

L _opt

optical length (optical path length)

f _rep

repetition

T

pulse duration

I

Intensity, irradiance

I _mean

mean intensity, mean irradiance

n

counting index

G

Just

N

Number of optical fibers

Δτ _max

maximum transit time difference

M

Average

Q

sum of squares

Claims (10)

Optical pulse broadening device ( 10 ) with (i) a plurality of optical fibers ( 12 ) of different optical lengths (L _opt ), (ii) a coupling device ( 14 ) for coupling a laser beam ( 20 ) in the light guides ( 12 ) and (iii) a decoupling device ( 16 ) for merging light rays from the light guides ( 12 ). Pulse broadening device ( 10 ) according to claim 1, characterized in that - the optical lengths (L _opt ), in particular the optical fiber lengths (L), have an optical fiber length distribution, - the optical fiber length distribution is a sum of squares (Q) in the form of the sum of the squares of Deviations from a given, in particular linear, distribution has and - by the number of light guides ( 12 ) divided root (σ) of the sum of squares is at most 0.5, in particular 0.1. Light source comprising (a) a pulsed laser adapted to emit light ( 20 ) having a repetition rate (f _rep ) and a pulse duration (T) and (b) a pulse widening device ( 10 ) according to one of the preceding claims, arranged so that the light ( 20 ) into the pulse broadening device ( 10 ) and thereby a pulse width broadening of the light ( 20 ) is achievable. Light source according to claim 3, characterized by a decoupling device ( 16 ) for coupling light ( 20 ) that the pulse broadening device ( 10 ) leaves, in a light guide ( 12 ), especially in a single-mode fiber. Light source according to one of claims 3 or 4, characterized in that the light guides ( 12 ) are formed so that a maximum transit time difference (Δτ _max ) between two optical fibers ( 12 ) is at least 0.8 times an inverse repetition rate (T _rep ). Light source according to one of claims 3 to 5, characterized in that at least one light guide is arranged so that at least a part of the light emerging from it is coupled again into the light guide and / or another light guide. Method for generating light ( 20 ), comprising the steps of: (a) generating pulsed laser light having a repetition rate and a pulse duration, (b) coupling the laser light into an optical pulse broadening device ( 10 ), which have a variety of light guides ( 12 ) of different optical lengths (L _opt ), so that a multiplicity of time-shifted partial light pulses is formed, and (c) uniting at least part of the partial light pulses into an output light beam ( 22 ). Method for generating a light path with a given time-varying frequency or wavelength, characterized in that light ( 20 ) is produced by the method according to claim 7, wherein laser light is used with the predetermined frequency. Method for calibrating an optical sensor, comprising the steps of: generating a light profile according to claim 8, determining a measured value which is an intensity of the light ( 20 ) at at least one point in time, - guiding the output light beam ( 22 ) on the optical sensor and - comparing the measured value with a sensor measured value recorded by the sensor. A method of manufacturing a pulse shaping device for shaping a light pulse so that it has a predetermined waveform (f (t)) at an interval between two light pulses, the light pulses having a time interval of a repetition time (T_rep), with a number (N) individual optical fibers having a length distribution (l (i)) assigning each optical fiber index (i) to the length (l), the length distribution being determined by the following method: Determining a parent function (F (t)) in the interval boundaries with the constraint that F (0) = 0, Normalizing the parent function (F (t)) such that F (T_rep) = N, From the inverse function (t_i = F ^ - 1 (i) with i = 1, ..., N) determining the delay times which must be achieved with the individual fibers, and - Assign a fiber length (l_i) at each delay time (t_i) according to the formula l_i = c · t_i / n, where c is the speed of light and n is the optical refractive index of the optical fiber.

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