JP6068946B2 - Mode-hop detector for wavelength-swept light source - Google Patents

Description

  The present invention relates to a technique for detecting a mode hop of a wavelength swept light source that sweeps the wavelength of emitted light continuously and periodically.

  As a light source capable of continuously sweeping a predetermined wavelength range among light sources capable of sweeping the wavelength of emitted light, an external resonance type wavelength tunable light source called a Litman type is known.

  As shown in FIG. 8, the external resonance type wavelength tunable light source is configured such that light emitted from one end face 1a of the semiconductor laser 1 enters the diffraction grating 2 at a predetermined incident angle and is diffracted from the diffraction grating 2 in a specific direction. Is reflected by the rotating mirror 3 to the diffraction grating 2 and returned to the semiconductor laser 1 by a reverse optical path.

  According to this structure, as shown in FIG. 9A, the semiconductor laser 1 is excited at an interval determined by the optical path length (resonator length) from the other end surface 1b having a high reflectance to the reflecting surface of the rotating mirror 3. Of the longitudinal mode light P (i), P (i + 1),... That is likely to be approximately the wavelength selection characteristic G determined by the incident angle of the light to the diffraction grating 2 and the angle of the rotating mirror 3 with respect to the diffraction grating 2 Only longitudinal mode light P (i + 3) entering the center is selectively excited. Then, by arranging each optical system so as to satisfy the condition that the wavelength selection characteristic G and the excitation mode are linked in the same direction as the angle of the rotation mirror 3 changes, the rotation mirror 3 is reciprocally rotated. The outgoing light wavelength can be continuously swept within a predetermined range. In this type of light source, as shown in FIG. 8, the light P that has passed through the end face 1b of the semiconductor laser 1 or the 0th-order diffracted light P ′ (shown by a dotted line) of the diffraction grating 2 is output to an external device. It will be used as light.

  In the wavelength tunable light source having such a structure, the characteristics of each optical element fluctuate due to a temperature change or the like, and the change amount of the wavelength selection characteristic A and the change amount of the excitation mode do not match during the sweep, and at a certain timing. 9B, the excited mode hops from the wavelength λ (i + 3) of the light P (i + 3) to, for example, the wavelength λ (i + 4) of the adjacent mode light P (i + 4). There is. This is called a mode hop. When this mode hop occurs, light having a wavelength between λ (i + 3) and λ (i + 4) is not output.

  Such a mode hop can be suppressed, for example, by adjusting the injection current of the semiconductor laser 1 or by controlling the temperature of the entire light source. To effectively perform the suppression control, the mode hop is used. A technology for correctly detecting the occurrence of this is required.

  As a technique for detecting this mode hop, a wavelength swept light is incident on a light receiver and its light intensity is detected, and a change width (time differentiation) of the light intensity per unit time is obtained. A technique is known in which a mode hop is considered to occur when the set threshold value is compared and the change width becomes equal to or greater than the threshold value (Patent Document 1).

JP 2010-212490 A

  However, since the above method detects a change in light intensity when a mode hop occurs, a correct determination cannot be made under conditions where there is no discernable difference between the light intensity before the mode hop and the light intensity after the mode hop. It was also difficult to grasp the degree of mode hops that occurred.

The present invention aims that the problem was solved, providing a mode hop detection equipment of the wavelength-swept light source that can be detected more accurately mode hopping.

In order to achieve the above object, a mode hop detection device for a wavelength swept light source according to claim 1 of the present invention comprises:
Optical interference that splits into two by receiving the outgoing light of the wavelength swept light source (20) that continuously and periodically sweeps the wavelength of the outgoing light within a predetermined range, and propagates through optical paths that are different in length by a predetermined length. Part (31);
A light receiver (32) for receiving the light combined by the interference unit;
A spectrum analyzer (35) for performing spectrum analysis on a beat signal appearing at an output of the light receiver due to a wavelength sweep by the wavelength swept light source and an optical path length difference of the optical interference unit;
The floor noise level of the beat signal obtained in the spectrum analysis unit, the floor noise level of the beat signal obtained in a state where no mode hop occurs in the wavelength swept light source, or the mode hop does not occur The floor noise level of the beat signal obtained in the state and the intermediate value of the floor noise level of the beat signal obtained in the state where the mode hop is generated are compared. A mode hop determination unit (36) for determination.

Further, the mode hop detection device for a wavelength swept light source according to claim 2 of the present invention is the mode hop detection device for a wavelength swept light source according to claim 1,
The mode hop determination unit
The floor noise level of the beat signal obtained by the spectrum analysis unit, the floor noise level of the beat signal obtained in the first state where the mode hop is generated by the wavelength swept light source, and the first state Comparing with an intermediate value of the floor noise level of the beat signal obtained in the second state where a larger mode hop occurs, and determining the degree of mode hop based on the comparison result To do.
Moreover, the mode hop detection device of the wavelength swept light source according to claim 3 of the present invention is the mode hop detection device of the wavelength swept light source according to claim 1 or 2,
The floor noise level of the beat signal used for determination by the mode hop determination unit is a relative value based on the peak level of the main spectrum of the beat signal.
Moreover, the mode hop detection device of the wavelength swept light source according to claim 4 of the present invention is the mode hop detection device of the wavelength swept light source according to claim 1 or 2,
The floor noise level of the beat signal used for determination by the mode hop determination unit is an absolute level.

  As described above, in the present invention, the spectrum of the beat signal obtained by splitting the light emitted from the wavelength swept light source into two branches, propagating through different optical paths and combining and inputting the light into the light receiver, Whether or not there is a mode hop is determined based on the floor noise level of the beat signal obtained by the spectrum analysis. The noise floor level of this beat signal corresponds to the phase fluctuation that occurs in the beat signal due to the mode hop. For example, even if the light intensity before and after the mode hop is the same, the wavelength sweep is caused by the phase fluctuation that occurs in the beat signal. The mode hop of the light source can be reliably detected. In addition, since the phase variation of the beat signal changes according to the mode hop level, the level of the mode hop can be grasped based on the magnitude of the noise floor level.

Configuration diagram of an embodiment of the present invention Operation explanatory diagram of the embodiment of the present invention Operation explanatory diagram of the embodiment of the present invention Operation explanatory diagram of the embodiment of the present invention Operation explanatory diagram of the embodiment of the present invention The figure which shows another structural example of this invention The figure which shows another structural example of the principal part of this invention Diagram showing a configuration example of a variable wavelength light source Illustration for explaining mode hops

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of a mode hop detection device 30 according to an embodiment that receives the sweep light emitted from the wavelength sweep light source 20 and detects the presence or absence of a mode hop.

  The structure of the wavelength swept light source 20 is arbitrary, but here, it is a Litman-type external resonator light source, and light emitted from one end face 22a of the semiconductor laser 22 is incident on the diffraction grating 23 at a predetermined incident angle. The diffracted light from the diffraction grating 23 with respect to the incident light is reflected by the rotating mirror 24.

  The rotating mirror 24 is a so-called MEMS type formed by an etching process on the semiconductor substrate, and is reciprocally rotated at a constant period by periodically applying a non-contact force by the driving circuit 25.

  The drive circuit 25 applies, for example, a high voltage signal synchronized with the input drive signal Sd between the end of the rotating mirror 24 and an electrode (not shown) opposed to and insulated from the end. Is periodically attracted by an electric field. Alternatively, the rotating mirror 24 may be reciprocally rotated at a constant period using a magnetic force generated between the coil and the magnetic body or permanent magnet, or between the coils.

  The light reflected by the rotating mirror 24 is incident on the diffraction grating 23 again. Here, of the diffracted light emitted from the diffraction grating 23 to the rotating mirror 24 side, only the light having the wavelength emitted in the direction orthogonal to the rotating mirror 24 is returned to the diffraction grating 23 through the same path, The diffraction grating 23 returns to the semiconductor laser 22.

  As described above, the wavelength swept light source 20 having this structure has wavelength selection characteristics determined by the incident angle of light to the diffraction grating 23 and the angle of the rotating mirror 24 with respect to the diffraction grating 23, and the reflectivity of the semiconductor laser 22 is improved. One of the excitation modes having a wavelength interval determined by the effective optical path length (resonator length) from the other high end face 22b to the reflecting surface of the rotating mirror 24 is at the center of the wavelength selection characteristic, and light of that wavelength is selectively selected. Excited. And each part is arrange | positioned so that the center wavelength of wavelength selection characteristic and excitation mode may interlock | cooperate with respect to the angle change of the rotation mirror 24, and it is an emitted light wavelength continuously according to the angle change of the rotation mirror 24. Is swept. Here, the emitted light P passing through the end face 22b of the semiconductor laser 22 of the wavelength swept light source 20 is used as light for mode hop detection, and the 0th-order diffracted light P ′ (shown by a dotted line) of the diffraction grating 23 or the light P The light branched by the branching means (not shown) is used as outgoing light for other external devices.

  In the case of the wavelength swept light source 20 having the above structure, as described above, there is a possibility that mode hops may occur due to fluctuations in the characteristics of the optical element due to temperature changes or the like.

  The mode hop detection device 30 for detecting the mode hop includes an optical interference unit 31, a light receiver 32, a spectrum analysis unit 35, and a mode hop determination unit 36.

  The optical interference unit 31 receives the outgoing light P from the wavelength swept light source 20, divides into two, propagates through optical paths having different lengths, and then multiplexes the optical interference unit 31. The configuration is arbitrary, but here, as an example, the structure of a Michelson interferometer is used.

  That is, the outgoing light P from the wavelength swept light source 20 is branched into two by a coupler (beam splitter) 31a, one of which is incident on a plane mirror 31b separated by a distance L1 and the reflected light P1 'is input to the coupler 31a through the same optical path. The other side P2 is incident on a plane mirror 31c separated by a distance L2 (> L1), the reflected light P2 'is returned to the coupler 31a through the same optical path, and both reflected lights P1' and P2 'are combined by the coupler 31a. Then, the light is emitted in a direction orthogonal to the incident direction. The reflected lights P1 ′ and P2 ′ propagate through different optical paths by a length of 2 (L2−L1) = ΔL.

  In the case of this configuration, the light is branched and multiplexed using the reflection characteristics and transmission characteristics at the boundary surface of the coupler 31a, but a part of the multiplexed light returns from the coupler 31a to the wavelength swept light source 20 side. Although the operation may be adversely affected, the isolator 29 provided on the end surface 22b side of the semiconductor laser 22 of the wavelength swept light source 20 prevents the light from returning to the wavelength swept light source 20 side.

  The light combined by the optical interference unit 31 enters the light receiver 32. As described above, when the incident light is branched into two and propagated through the optical path having the optical path length difference ΔL, if L2> L1, the wavelength with respect to time of one light P1 ′ incident on the light receiver 32 is For example, if it changes according to the characteristic A in FIG. 2A, the wavelength of the other light P2 ′ with respect to the time is equal to the time Δt (= ΔL / c) of the characteristic A corresponding to the optical path length difference ΔL. It changes according to the characteristic B shifted (delayed) on the axis.

  When the light P1 'and P2' whose wavelength change characteristics are shifted in this way are incident on the light receiver 32, the light receiver 32 changes at a frequency equal to the frequency difference Δf between the light P1 'and P2'. Beat signal to be output.

  The frequency Δf of the beat signal changes depending on the magnitude of the deviation of the characteristics A and B on the time axis Δt = ΔL / c (c is the speed of light), that is, the optical path length difference ΔL, and the optical path length difference ΔL is short. And becomes higher as the optical path length difference ΔL becomes longer. That is, the beat frequency Δf can be expressed as a function using the optical path length difference ΔL.

Now, consider the case where the angular frequency of light changes linearly with time,
ω (t) = ω ₀ + 2πV · t
(Ω ₀ is the initial value, V is the slope)
Then, the angular frequency of the light delayed by Δt is
ω (t) = ω ₀ + 2πV · (t−Δt)
And the angular frequency component Δω of the difference from the light receiver 32,
Δω = [ω ₀ + 2πV · t] − [ω ₀ + 2πV · (t−Δt)]
= 2πVΔt = 2πV · ΔL / c
A beat signal with is output.

That is, the frequency Δf of the beat signal in a range where the change in the angular frequency of the light can be regarded as a straight line is
Δf = Δω / (2π) = V · ΔL / c (1)
It is represented by

  However, the actual wavelength sweep characteristic is a sine function as shown in FIG. 2A, and the angular frequency of light inversely proportional to it is also a nonlinear function, but Δt is sufficiently small and linear approximation is performed. The above calculation is valid in a narrow region where it is possible.

  Therefore, if the characteristics A and B are sine functions as described above, the frequency Δf of the beat signal changes as shown in FIG. That is, at the timings t1 and t3 when the characteristics A and B intersect, the frequency difference of the light becomes zero, the signal output becomes a direct current as shown in FIG. 2 (c), and the frequency gradually increases from the timing t1 at the intermediate point t2. The maximum frequency is reached and gradually decreases until timing t3. Further, the waveform of FIG. 2C is shown by paying attention only to the frequency of the beat signal, and it is assumed that the level change becomes smaller as it becomes closer to DC.

In FIG. 2, the time difference Δt between the characteristics A and B is greatly expressed so that the beat signal generation process can be easily understood. However, the time difference Δt = L / c caused by the optical path length difference L is, for example, L = 3 mm. , C (speed of light) = 3 × 10 ⁸ m, Δt = 10 × 10 ⁻¹² = 10 ps, whereas the sweep period T is in the range of several ms to several hundred μs with a variable wavelength light source having a MEMS structure. Since the ratio between the two is 10,000 times or more, it can be considered that the characteristics A and B overlap with each other even when there is no time difference Δt when viewed from the sweep cycle T. The time for light to propagate through several millimeters to several centimeters is negligible compared to the period of the rotating mirror. Here, the time for the incident light to reach the light receiver 32 is also the drive signal and wavelength. It can be ignored as information for specifying the relationship.

  The beat signal whose frequency changes during the sweep is input to the spectrum analysis unit 35.

  The spectrum analysis unit 35 performs spectrum analysis on the beat signal appearing at the output of the light receiver 32, but the frequency of the beat signal varies with time within the wavelength sweep period as shown in FIG. Here, the A / D converter 35a samples the beat signal at a frequency sufficiently higher than the upper limit frequency for at least a half cycle of the wavelength sweep (for example, from time t1 to time t3 in FIG. 2), and the waveform data is sampled. Store in the memory 35b.

  Then, the analysis unit 35c cuts out the stored waveform data for each short time period in which the frequency can be regarded as substantially constant, performs FFT calculation processing on the data in each time period, and obtains spectrum characteristics for each time period. .

  Here, in the time zone when the mode hop does not occur, the frequency and amplitude of the beat signal are almost constant and the phase continuity is maintained as shown in FIG. As shown in 3 (b), the peak of the main spectrum is large, and the average value FN0 of the floor noise level in a predetermined band (preset) on both sides thereof is low. Here, the floor noise level and its average value are relative values with the peak level of the main spectrum as a reference (0 dB), but may be absolute levels.

  Further, in a time zone in which a mild mode hop that hops to another excitation wavelength for a moment and returns to the original wavelength occurs, an instantaneous phase change occurs in the beat signal as shown in FIG. Since a spike-like change accompanying this appears, the spectrum characteristic shows that the peak of the main spectrum is slightly lower than the state of FIG. 3, as shown in FIG. 4B, and the average value of the floor noise levels on both sides thereof. FN1 is higher than FN0.

  Further, in a time zone in which a mode hop that completely hops to another excitation wavelength occurs, the phase of the beat signal changes discontinuously as shown in FIG. As shown in FIG. 5B, the characteristic is that the peak of the main spectrum is further lowered as compared with the state of FIG. 4, and the average value FN2 of the floor noise levels on both sides is further higher than FN1.

  As described above, the floor noise level changes according to the mode hop occurrence state. Therefore, by monitoring the floor noise level, it is possible to grasp the presence and degree of the mode hop.

  The mode hop determination unit 36 monitors the floor noise level of the spectrum characteristic for each time zone obtained by the spectrum analysis unit 35, and the reference floor noise level for each time zone in the state where there is no mode hop and the current floor. The presence or absence of a mode hop is determined by comparing with the noise level.

  Here, when simply determining whether or not there is a mode hop, the current injected into the semiconductor laser 22 is varied in advance to artificially generate a mode hop, and the floor noise level FN0 in the states of FIGS. The intermediate value of FN1 may be used as a threshold value for presence / absence determination.

  When determining not only the presence / absence of a mode hop but also the degree (frequency), for example, an intermediate value R1 of the floor noise levels FN0 and FN1 (also for determining the presence / absence) and an intermediate value R2 of the floor noise levels FN1 and FN2 Is “no mode hop” if it is less than or equal to the threshold value R1, “mode hop is small” if it is greater than or equal to R1 and less than R2, and “mode hop is large” if it is greater than or equal to the threshold value R2. What is necessary is just to judge.

  The determination result by the mode hop determination unit 36 is displayed on the display unit 40, for example. The user can grasp the presence or absence and the degree of the mode hop from the display result. For example, when the mode hop is generated, the semiconductor laser 22 is displayed on the display unit 40 so that the mode hop is not generated. Adjust the injection current.

  In this embodiment, an example of the mode hop detection device 30 that is externally attached to the wavelength swept light source 20 and displays the presence or absence and the degree of the mode hop is shown. However, as shown in FIG. The function of the detection device 30 is built in the wavelength sweep light source 20 ′, and a control unit 50 is provided that variably controls the injection current of the semiconductor laser 22 according to the determination result of the mode hop determination unit 36 and suppresses the generation of mode hops. May be.

  In the above embodiment, the configuration of the optical interference unit 31 is a Michelson interferometer. However, if the optical interference unit 31 receives an incident light and splits into two and propagates through optical paths having different lengths, the configuration Is optional. However, it is required that the variation in the optical path length difference ΔL due to environmental changes is small. Therefore, for example, instead of using the plane mirrors 31b and 31c in the above-described embodiment, as shown in FIG. 7, a part P1 of the light emitted from the coupler 31a (beam splitter) is changed to one surface side of the plate 31f having a constant thickness. The light P2 reflected by the surface 31g is reflected by the opposite inner surface 31h so that the reflected light P1 'and P2' are returned to the coupler 31a. The thickness d of the plate 31f An optical path length difference ΔL corresponding to twice this value may be given.

  DESCRIPTION OF SYMBOLS 20 ... Wavelength sweep light source, 22 ... Semiconductor laser, 23 ... Diffraction grating, 24 ... Rotating mirror, 25 ... Drive circuit, 30 ... Mode hop detector, 31 ... Optical interference part, 32 ... Light receiver, 35 ... Spectrum analysis unit, 36 ... Mode hop determination unit, 40 ... Display unit, 50 ... Control unit

Claims (4)

Optical interference that splits into two by receiving the outgoing light of the wavelength swept light source (20) that continuously and periodically sweeps the wavelength of the outgoing light within a predetermined range, and propagates through optical paths that are different in length by a predetermined length. Part (31);
A light receiver (32) for receiving the light combined by the interference unit;
A spectrum analyzer (35) for performing spectrum analysis on a beat signal appearing at an output of the light receiver due to a wavelength sweep by the wavelength swept light source and an optical path length difference of the optical interference unit;
The floor noise level of the beat signal obtained in the spectrum analysis unit, the floor noise level of the beat signal obtained in a state where no mode hop occurs in the wavelength swept light source, or the mode hop does not occur The floor noise level of the beat signal obtained in the state and the intermediate value of the floor noise level of the beat signal obtained in the state where the mode hop is generated are compared. A mode hop detection device for a swept wavelength light source, comprising a mode hop determination unit (36) for determination. The mode hop determination unit
The floor noise level of the beat signal obtained by the spectrum analysis unit, the floor noise level of the beat signal obtained in the first state where the mode hop is generated by the wavelength swept light source, and the first state Comparing with an intermediate value of the floor noise level of the beat signal obtained in the second state where a larger mode hop occurs, and determining the degree of mode hop based on the comparison result The mode hop detection device for a swept light source according to claim 1. 3. The wavelength swept light source according to claim 1, wherein the floor noise level of the beat signal used for the determination by the mode hop determination unit is a relative value based on the peak level of the main spectrum of the beat signal. Mode hop detection device. 3. The mode hop detection device for a wavelength swept light source according to claim 1, wherein the floor noise level of the beat signal used for determination by the mode hop determination unit is an absolute level.

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