JP2000353854A - External-resonator type variable-wavelength light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable wavelength to be stably varied within a wavelength band, where laser oscillation of an optical amplifier is possible by avoiding the occurrence of mode hopping in an external-resonator type variable-wavelength light source, even when operating conditions change. SOLUTION: This external-resonator type variable-wavelength light source is equipped with an optical amplifier 1, one end face of which has non-reflecting film 1a, a diffraction grating 2 which has a wavelength selection function positioned on the exiting optical axis on the non-reflecting film 1a side, and a reflector 42 which reflects a first-order diffraction light L1 toward the diffraction grating 2. This light source is also equipped with a first photodetector 43, which receives light that comes from the optical amplifier 1 to the diffraction grading 2 and is reflected as a zero-th-order diffraction light L0, and converts the light into an electrical signal, an optical branch unit 41 which branches a forward output light Lf of the optical amplifier 1, that is emitted from the end face having no non-reflecting film 1a, a second photodetector 44 which receives the branched light Ls and converts it into an electrical output, and a processing circuit 45 which calculates electrical outputs from the first photodetector unit 43 and the second photodetector 44. The output value from the processing circuit 45 is fed back to a phase adjustment means 46 of the reflector 42 so that it is adjusted to a prescribed value, thereby varying the wavelength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external resonator type variable wavelength light source used in the field of optical communication and optical measurement.

[0002]

2. Description of the Related Art A light source used in optical measurement technology is required to be a single mode oscillation having a narrow spectral line width, having good wavelength stability, and capable of tunable wavelength. Then, an external resonator type wavelength tunable light source using a semiconductor laser device having a wide gain range and a diffraction grating having a wavelength selection function has been developed and used.

FIG. 7 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a first conventional example. The external resonator type wavelength tunable light source includes an optical amplifier 1, a diffraction grating 2,
It comprises lenses 3, 4, an optical isolator 5, an optical amplifier drive circuit 6, a rotating mechanism 7, a parallel moving mechanism 8, and the like. The optical amplifier 1 is a Fabry-Perot type semiconductor laser, has a non-reflective film 1 a on one end face, and emits light from both end faces according to an injection current from the optical amplifier drive circuit 6.
The lens 3 is disposed on the emission optical axis on the side of the non-reflective film 1a of the optical amplifier 1, and converts the light emitted from the end surface of the optical amplifier 1 on the side of the non-reflective film 1a into parallel light. The emitted light converted into parallel light in this way is incident on the diffraction grating 2.

[0004] The diffraction grating 2 is used as a wavelength selective reflector and has a function of reflecting light of a specific wavelength determined by the incident angle among incident parallel lights in the direction of the incident optical axis.
Further, the diffraction grating 2 forms a resonator with the end face of the optical amplifier 1 on which the non-reflection film 1a is not applied, and the light selected by the diffraction grating 2 is re-entered into the optical amplifier 1 so that
Laser oscillation is possible. Lens 4
Is disposed on the emission optical axis on the side of the optical amplifier 1 on which the antireflection film 1a is not applied, and converts light emitted from the end face of the optical amplifier 1 into parallel light. The emitted light converted into parallel light in this manner is incident on the optical isolator 5. The optical isolator 5 reflects the light reflected from the output fiber 11 side to the optical amplifier 1.
The light transmitted through the optical isolator 5 is condensed by a lens 9 and is incident on an output fiber 11 as output light.

The wavelength of the external resonator type wavelength tunable light source is determined by two conditions (resonance wavelength λFP and Bragg wavelength λB), and the principle of the wavelength determination is shown in FIG. As shown in FIG. 5, the reflectance of the diffraction grating 2 with respect to the wavelength has wavelength selectivity, so that the reflectance changes with the wavelength. The end face of the optical amplifier 1 on which the anti-reflection film 1a is not applied has a constant reflectance (about 30%) without wavelength characteristics because the cleavage face of the semiconductor laser is used. And
The resonance wavelength λFP as the phase matching condition by the resonator is
It is shown by equation (1). λFP = 2 × n × L / M (1) where λFP is the resonance wavelength of the resonator, L is the length of the resonator,
M indicates the number of resonance longitudinal modes of the resonator. for that reason,
Many resonance wavelengths λFP exist as resonance longitudinal modes.

The Bragg wavelength selected by the optical arrangement (retro arrangement) of the optical amplifier 1 and the diffraction grating 2 shown in FIG. 7 is expressed by the following equation (2). λB = 2 × d × sin θ / N (2) where λB is the Bragg wavelength selected by the diffraction grating 2,
d is the groove interval of the diffraction grating 2, N is the order of the diffracted light (usually N =
1) and θ indicate the angle between the normal line of the diffraction grating 2 and the emission optical axis of the optical amplifier 1 (the angle of incidence on the diffraction grating 2). The black wavelength λB functions as a filter for selecting one longitudinal mode from the resonance wavelengths λFP of the above-mentioned many resonance longitudinal modes. In other words, the laser oscillation of the external resonator type wavelength tunable light source oscillates at the resonance wavelength λFP where the mirror loss near the black wavelength λB selected by the diffraction grating 2 is small and the phase matching condition by the resonator is satisfied.

The diffraction grating 2 has a rotating mechanism 7 so that the angle with respect to the incident optical axis can be adjusted to an arbitrary angle (θ).
Only the diffracted light having the wavelength determined by equation (2) is diffracted into the same optical path as the incident optical path, is fed back to the optical amplifier 1, and is oscillated by the laser. Light other than the wavelength determined by the equation (2) is diffracted into an optical path different from the incident optical path.
It does not return to the optical amplifier 1. The wavelength varying means 12 includes a rotation mechanism 7 for varying the angle of the diffraction grating 2 and a rotation control means 13.
, A rotation control drive circuit 14, a parallel movement mechanism 8 for varying the position, a parallel movement control means 15, and a parallel movement drive circuit 16.
It is composed of The rotation mechanism 7 is configured to rotate by driving the rotation control means 13 based on a signal from the rotation control drive circuit 14. Then, by controlling the rotation mechanism 7 by the rotation control drive circuit 14, the diffraction grating 2 can be rotated to an arbitrary angle and the selected wavelength (Bragg wavelength) can be changed arbitrarily. Wavelength can be tuned within the gain range described above.

The diffraction grating 2 is disposed on the translation mechanism 8 including the rotation mechanism 7 so that the diffraction grating 2 can be translated in the optical axis direction by the translation mechanism 8. Then, by controlling the translation mechanism 8 by the translation drive circuit 16, the diffraction grating 2 can be translated in the optical axis direction of the resonator, and thereby the resonance wavelength can be changed. Therefore, the wavelength of the external resonator wavelength tunable light source can be tuned by separately adjusting the Bragg wavelength λB and the resonance wavelength λFP. However, if only the angle of the diffraction grating 2 changes, only the Bragg wavelength changes and the resonance wavelength does not change, so that the wavelength changes with mode hops. Similarly, the resonance wavelength changes only by the parallel movement of the diffraction grating 2 in the optical axis direction, but the Bragg wavelength λB
Does not change, the resonance wavelength λF near the Bragg wavelength λB
With the change of P, the mode hop is repeated.

In the case of wavelength tuning without mode hop, the same wavelength amount is simultaneously changed by changing the Bragg wavelength λB selected by the diffraction grating 2 and the resonance wavelength λFP by the number of M-order longitudinal modes of the resonator. It is necessary to change the wavelength in a certain longitudinal mode. However, it is extremely difficult to control the change of the incident angle and the change of the resonator length individually and continuously over a wide range to perform wavelength tuning without mode hop. Further, the refractive index of the semiconductor laser device as the optical amplifier 1 changes depending on the drive current, temperature, and wavelength. The change in the refractive index with respect to the drive current and the temperature is almost linear. However, in a semiconductor laser device, the refractive index changes with the carrier density inside the semiconductor laser (plasma effect), and the threshold carrier density is non-linear with respect to wavelength, so that the refractive index with respect to wavelength is also non-linear.

Even if the physical resonator length and the Bragg wavelength of the diffraction grating 2 are accurately and linearly changed in order to perform wavelength tuning without mode hopping, the actual optical resonator length changes non-linearly, and furthermore, the mode hopping changes. It becomes difficult to change the wavelength without using. Furthermore, since the rotation mechanism 7 and the parallel movement mechanism 8 involve mechanical backlash, there is a problem that accurate control cannot be performed by control without feedback control.

By the way, as an improved type of the wavelength variable means 12 which individually controls the change of the incident angle to the diffraction grating 2 and the change of the resonator length by the first prior art external resonator type wavelength variable light source, U.S. Pat. No. 5,347, U.S. Pat.
527). That is, as shown in FIG. 8, the wavelength of the sine bar 17 mechanism can be changed only by drive control by the parallel movement drive circuit 16 so that the angle change of the diffraction grating 2 and the resonator length change can be performed simultaneously. Wavelength tuning with few mode hops can be easily performed. However, as described above, the refractive index of the semiconductor laser device as the optical amplifier 1 changes depending on the drive current, temperature, and wavelength. Therefore, even if the wavelength is changed beyond a certain wavelength range even when controlled by the sine bar 17 mechanism. Then, the resonance wavelength λFP and the Bragg wavelength λB deviate, and the wavelength becomes variable with mode hop.

Next, FIG. 9 shows the configuration of a second conventional external resonator type wavelength tunable light source. The external resonator type wavelength tunable light source includes an optical amplifier 1, a diffraction grating 2, a lens 3,
4, optical isolator 5, optical amplifier drive circuit 6, reflector 2
2. It is composed of a rotation mechanism 23 and the like. In FIG. 9, the same parts as those in FIG.
The description is omitted. The Bragg wavelength selected by the optical arrangement (Littman arrangement) of the optical amplifier 1, the diffraction grating 2, and the reflector 22 is expressed by the following equation (3). .lambda.B = d / N.times. [sin (.alpha.) + sin (.beta.)] (3) where .alpha. is the angle between the normal to the diffraction grating 2 and the emission optical axis of the optical amplifier 1 (to the diffraction grating 2). Incident angle), β is diffraction grating 2
Respectively, and the angle (reflection angle from the diffraction grating 2) between the normal line and the reflection optical axis of the light diffracted by the diffraction grating 22.

The diffraction grating 2 is a non-reflective film 1a of the optical amplifier 1.
The light emitted from the side end surface is arranged at a position where the light is incident at an incident angle α, and is fixed to the optical base 10. Reflector 2
Numeral 2 is disposed on the rotating mechanism 23, and reflects only light of a wavelength that is perpendicularly incident on the reflector 22 out of the diffracted light from the diffraction grating 2 obtained by the equation (3) into the same optical path as the incident optical path. Then, the laser beam is fed back to the optical amplifier 1 to cause laser oscillation. Light having a wavelength not perpendicularly incident on the reflector 22 is reflected on an optical path different from the incident optical path, and therefore does not return to the optical amplifier 1. The rotation mechanism 23 is configured to rotate around a fulcrum by driving the rotation control means 21 under the control of the rotation control drive circuit 26, and with the rotation, changes in the angle of the reflector 22 and changes in the resonator length. At the same time to change the black wavelength λB and the resonance wavelength λFP. Therefore, by designing the rotation center position of the reflector 22 to be the optimum position, the wavelength can be easily changed with almost no mode hop. However, as in the first prior art, the refractive index of the semiconductor laser device, which is the optical amplifier 1, changes depending on the drive current, temperature, and wavelength. Is changed, the resonance wavelength .lambda.FP and the Bragg wavelength .lambda.B deviate, and the wavelength becomes variable with mode hop.

Next, as a third conventional example, FIG. 10 shows a control block diagram of an external resonator type wavelength tunable light source (refer to Japanese Patent Application Laid-Open No. 9-102645) using a control means for preventing occurrence of mode hop. . The control means 31 includes an optical splitter in a resonator of an end surface of the optical amplifier (semiconductor laser) 1 on the side of the non-reflection film 1a and a reflection device 32 having a wavelength selectivity using a diffraction grating (wavelength selection element) 2 or the like. (Diplexer) 33 is inserted, and the first light receiver 34 and the second light receiver 35 are used to compare the light intensity emitted from the optical amplifier 1 with the light intensity returned from the reflection device 31 to the optical amplifier 1. Control is performed so that the ratio becomes a predetermined value. According to the description in the publication, the light returning from the reflection device 31 to the optical amplifier 1 includes a diffraction grating (first light).
Of the filter characteristic of the optical amplifier (the second reflector) 2 and the light that does not include the information of the wavelength selectivity such as light emitted from the optical amplifier (second reflector) 1. It can be controlled by comparison.

However, when the wavelength of laser oscillation is
If the wavelength changes to the side where the reflectance of the optical amplifier 1 decreases, the light intensity returning to the optical amplifier 1 decreases, but the mirror loss increases and the light intensity of laser oscillation also decreases. Conversely, when the wavelength changes to the side where the reflectance increases, the light intensity returning to the optical amplifier 1 increases, but the mirror loss decreases and the light intensity of laser oscillation also increases. Therefore, the intensity change of the light returning from the reflection device 32 to the optical amplifier 1 and the intensity change of the light exiting from the optical amplifier 1 change in the same direction. Optical amplifier 1
In the ratio between the light intensity returning to the optical amplifier 1 and the light intensity emitted from the optical amplifier 1, a sufficient detection amount cannot be obtained, and it is difficult to perform control so as to prevent the occurrence of mode hop.

[0016]

As described above, conventionally,
In the external resonator type wavelength tunable light source, there is a problem that when the wavelength is tunable under various use conditions (temperature, drive current), the wavelength tunable width without a mode hop is reduced.

It is an object of the present invention to provide an image forming apparatus which
An object of the present invention is to provide an external resonator type wavelength tunable light source capable of tunable wavelength while preventing occurrence of mode hop.

[0018]

In order to solve the above-mentioned problems, the invention according to claim 1 is, for example, an optical amplifier (FIG. 1) having one end face provided with a non-reflection film (1a). 1)
A diffraction grating (2) arranged on the optical axis on the antireflection film side of the optical amplifier and having a wavelength selecting function; and a first-order diffracted light (L1) arranged on the optical axis of the first-order diffracted light of the diffraction grating. And a reflector (42) that reflects light again to the diffraction grating, and receives the 0th-order diffracted light (L0) reflected at an angle symmetrical to the incident angle with respect to the diffraction grating. A first photodetector (43) for converting into an electric signal, and a forward output light (Lf) emitted from the other end face of the optical amplifier on which the antireflection film is not applied are output light (L) and split light (Ls). ) And a second optical receiver (4) that receives the split light from the optical splitter and converts the split light into an electric signal.
4), an arithmetic processing circuit (45) for performing predetermined arithmetic processing on the electric signals from both the first and second optical receivers, and a reflector (4).
2) a phase adjusting means (46) for adjusting the position, and a wavelength variable means (47) for performing feedback control to the phase adjusting means so as to control the output value from the arithmetic processing circuit to a predetermined value and variably controlling the wavelength. And a configuration comprising:

As described above, according to the first aspect of the present invention, the light intensity of the zero-order diffracted light is measured by the first light receiver as the light intensity of the backward output light of the optical amplifier, and the forward output of the optical amplifier is measured. The light intensity of the light is measured by the second light receiver, and the intensity ratio between the forward output light emitted in the forward direction and the backward output light emitted to the diffraction grating side is defined as the Bragg wavelength (λB) and the resonance wavelength (λFP). The amount of deviation is obtained by an arithmetic processing circuit, and the wavelength is tunable by the wavelength tunable means while performing feedback control of the position or angle of the reflector by the phase adjusting means so that the output value from the arithmetic processing circuit becomes constant. , A wavelength tunable without mode hop is obtained.

According to a second aspect of the present invention, there is provided an external resonator type wavelength tunable light source according to the first aspect, wherein, for example, as shown in FIG. 2, the phase adjusting means (46) includes a reflector (42). An arithmetic processing circuit comprising: a parallel movement mechanism (48) for holding; a parallel movement control means (54) for controlling the position of the parallel movement mechanism; and a parallel movement drive circuit (55) for driving the parallel movement control means. The control signal is output from the parallel movement drive circuit in response to the electric signal from the controller and the parallel movement control means is driven to control the position of the reflector (42) with respect to the reflected optical axis.

According to a third aspect of the present invention, there is provided an external resonator type wavelength tunable light source according to the first aspect, wherein, as shown in FIG. 3, the phase adjusting means (46) includes a reflector (42). A second rotation mechanism (58) for holding, a second rotation control means (57) for controlling the angle of the second rotation mechanism, and a second rotation control drive circuit for driving the second rotation control means (56), a control signal is output from the second rotation control drive circuit in response to an electric signal from the arithmetic processing circuit to drive the second rotation control means, so that the light is reflected with respect to the reflected optical axis. Controlling the angle of the vessel (42).

Further, the invention according to claim 4 provides, for example, an optical amplifier (1) having one end face provided with a non-reflection film (1a) as shown in FIG. Grating arranged on the optical axis on the side and having a wavelength selection function (2)
A wavelength-variable light source of an external resonator type, comprising: a first light-receiving element that receives the 0th-order diffracted light (L0) reflected at an angle symmetrical with respect to the incident angle with respect to the diffraction grating (2) and converts the light into an electric signal; And an optical splitter (43) for splitting the forward output light (Lf) emitted from the other end face of the optical amplifier on which the non-reflection film is not applied, into output light (L) and split light (Ls). 41)
A second photodetector (44) for receiving the split light from the optical splitter and converting the split light into an electric signal; and an arithmetic processing for performing predetermined arithmetic processing on the electric signals from both the first and second photodetectors. A circuit (45), a phase adjusting means (46) for adjusting the position of the diffraction grating (2), and a feedback control to the phase adjusting means so as to control the output value from the arithmetic processing circuit to a predetermined value to adjust the wavelength. And a variable wavelength control means (47) for variably controlling.

As described above, according to the invention, the light intensity of the 0th-order diffracted light is measured by the first light receiver as the light intensity of the backward output light of the optical amplifier, and the forward output of the optical amplifier is measured. The light intensity of the light is measured by the second light receiver, and the intensity ratio between the forward output light emitted in the forward direction and the backward output light emitted to the diffraction grating side is defined as the Bragg wavelength (λB) and the resonance wavelength (λFP). The amount of deviation is obtained by the arithmetic processing circuit, and the wavelength is tunable by the wavelength tunable means while performing feedback control of the position of the diffraction grating by the phase adjusting means so that the output value from the arithmetic processing circuit becomes constant. A hop-free wavelength tunable is obtained.

According to a fifth aspect of the present invention, there is provided an external resonator type wavelength tunable light source according to the fourth aspect. For example, as shown in FIG. 4, the position adjusting means (46) includes a diffraction grating (2). An arithmetic processing circuit comprising: a parallel movement mechanism (48) for holding; a parallel movement control means (54) for controlling the position of the parallel movement mechanism; and a parallel movement drive circuit (55) for driving the parallel movement control means. The control signal is output from the parallel movement drive circuit in response to the electric signal from the controller and the parallel movement control means is driven to control the position of the diffraction grating (2) with respect to the emission optical axis.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of an external resonator type wavelength variable light source according to the present invention will be described below with reference to FIGS.

<First Embodiment> FIG. 1 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a first embodiment of the present invention. As shown in the figure, the external resonator type wavelength tunable light source includes a semiconductor laser 1 as an optical amplifier, a diffraction grating 2, lenses 3 and 4 arranged on an emission optical axis of the optical amplifier 1, an optical isolator 5, an optical amplifier. It comprises a drive circuit 6, an optical splitter 41, a reflector 42, a first light receiver 43, a second light receiver 44, an arithmetic processing circuit 45, a wavelength varying means 46, a phase adjusting means 47 and the like. The optical amplifier 1 is a Fabry-Perot type semiconductor laser, and has an antireflection film 1a on one end surface. Further, the optical amplifier 1 is connected to the optical amplifier drive circuit 6 and emits light from both end surfaces according to the injection current from the optical amplifier drive circuit 6. The lens 3 is disposed on the emission optical axis on the side of the non-reflective film 1a of the optical amplifier 1, and converts the light emitted from the end surface of the optical amplifier 1 on the side of the non-reflective film 1a into parallel light. The emitted light converted to the parallel light is incident on the diffraction grating 2.

The diffraction grating 2 is a non-reflective film 1a of the optical amplifier 1.
The emitted light is arranged at a position where the emitted light is incident on the optical axis from the side end surface at an incident angle α. The diffraction grating 2 functions as a wavelength-selective reflector, and converts most (reflectance) of the light having the wavelength represented by the above-described equation (3) among the parallel light incident from the optical amplifier 1 at the incident angle α. The light is diffracted at an angle β as the first-order diffracted light L1. On the other hand, the light that is not diffracted by the light having the wavelength represented by the equation (3) is converted into an incident angle α with respect to the normal to the diffraction grating 2.
And is reflected as the 0th-order diffracted light L0 at an angle symmetrical to Also,
The lens 4 is disposed on the emission optical axis of the optical amplifier 1 on the side where the non-reflection film 1a is not applied, and converts the forward output light Lf emitted from the end face of the optical amplifier 1 into parallel light. The forward output light Lf converted into the parallel light is incident on the optical isolator 5. The optical isolator 5 is for preventing a part of the forward output light Lf that has passed through the optical isolator 5 from returning to the optical amplifier 1 as reflected light. Enters the optical splitter 41. The optical splitter 41 splits the incident light of the forward output light Lf into split light Ls and output light L.

The reflector 42 is provided between the optical amplifier 1 and the diffraction grating 2.
And is arranged at a position where the first-order diffracted light L1 having the wavelength diffracted at the angle β is incident, and reflects the first-order diffracted light L1.
At this time, of the diffracted light from the diffraction grating 2 obtained by the equation (3), only light having a wavelength that is perpendicularly incident on the reflector 42 is reflected on the same optical path as the incident optical path, and is returned to the optical amplifier 1. ,
Start laser oscillation. Note that light having a wavelength not perpendicularly incident on the reflector 42 is reflected on an optical path different from the incident optical path, and therefore does not return to the optical amplifier 1. The first light receiver 43 receives the light reflected as the 0th-order diffracted light L0 from the diffraction grating 2, converts the light into an electric output, and outputs the electric output to the arithmetic processing circuit 45 as the light intensity on the diffraction grating 2 side. The second light receiver 44 is
The split light Ls from the optical splitter 41 enters, is converted into an electrical output, and is output to the arithmetic processing circuit 45 as the light intensity of the forward output light Lf. The arithmetic processing circuit 45 calculates the electric output input from the first light receiver 43 and the second light receiver 44, obtains the ratio between the intensity of the forward output light Lf and the intensity of the output light on the diffraction grating 2 side, and outputs a control signal. And outputs it to the phase adjusting means 46.

Next, FIG. 2 is a block diagram for explaining the wavelength changing means 47 and the phase adjusting means 46 of FIG. The reflector 42 is arranged on a parallel moving mechanism 48 as a phase adjusting means 46, and further arranged together with the parallel moving mechanism 48 on a rotating mechanism 49 as a wavelength changing means 47.
The wavelength variable means 47 includes a rotation mechanism 49 for changing the angle of the reflector 42 and changing the resonator length, a rotation control means 51, and a rotation control drive circuit 52. The rotation mechanism 49
It is configured to rotate around a fulcrum 53 by driving of the rotation control means 51 under the control of the rotation control drive circuit 52,
Along with the rotation, the angle change of the reflector 42 and the change of the resonator length are simultaneously performed to change the Bragg wavelength λB and the resonance wavelength λFP.

The phase adjusting means 46 includes a translation mechanism 48 for changing the reflector 42 in the optical axis direction and a translation control means 5.
4 and a parallel movement drive circuit 55. The translation mechanism 48 is configured to translate by driving the translation control means 54 based on a signal from the translation drive circuit 55, and can slightly change the resonance wavelength λFP. The translation drive circuit 55 drives the translation control means 54 to control the position of the reflector 42 such that the value of the control signal input from the arithmetic processing circuit 45 becomes a predetermined value. In the configuration of the first embodiment, the resonance wavelength λFP is minutely controlled with respect to the change in the Bragg wavelength λB by the wavelength variable means 47.

As described above, according to the external resonator type wavelength tunable light source of the first embodiment, the 0th-order diffracted light L0 from the diffraction grating 2 is obtained as the light intensity of the backward output light Lr of the optical amplifier 1.
Is measured by the first light receiver 43, the light intensity of the forward output light Lf of the optical amplifier 1 is measured by the second light receiver 44, and the forward output light Lf emitted in the forward direction is diffracted. The intensity ratio of the backward output light Lr emitted to the grating 2 is represented by the Bragg wavelength λ.
The difference between B and the resonance wavelength λFP is obtained by the arithmetic processing circuit 45.

Next, the operation principle of controlling the generation of the mode hop will be described with reference to the light output characteristics with respect to the wavelength when the wavelength is changed while the mode hop is performed in the above-described external resonator type wavelength tunable light source. FIG. 6 shows the light output characteristics when the change rate of the change in the resonance wavelength due to the change in the resonator length and the change in the black wavelength due to the change in the angle of the diffraction grating 2 is different (when a mode hop occurs). That is,
As shown in FIG. 6, the optical output characteristic has a sawtooth output change with respect to the wavelength, and a mode hop occurs at the wavelength of the sawtooth discontinuous change. Further, the forward output light Lf
And the intensity change of the 0th-order diffracted light L0 show opposite phase changes.

As described above, when a semiconductor laser element is used for the optical amplifier 1, the refractive index changes due to the change in the carrier density inside the semiconductor laser (plasma effect). In the plasma effect, as the carrier density increases, the wavelength changes to the shorter wavelength side. Therefore, when the reflectance of the reflector 42 or the diffraction grating 2 decreases, the mirror loss increases and the threshold carrier density increases. As described above, when the carrier density increases, the wavelength changes to the short wavelength side. However, on the short wavelength side due to the reflectance characteristic of the diffraction grating 2, when the wavelength tries to change to the short wavelength side, the reflectance decreases. The carrier density increases, the wavelength further shifts to the shorter wavelength side, and the reflectance decreases, so that laser oscillation cannot be performed. Conversely, on the long wavelength side, when the wavelength tries to change to the long wavelength side, the reflectance decreases and the carrier density increases, and a feedback effect that the wavelength tries to change to the short wavelength side is obtained. This results in laser oscillation. Therefore, the sawtooth output change occurs because the laser oscillation occurs only in the wavelength region of the long wavelength side in the reflection characteristic of the diffraction grating 2.

Further, there is a periodic change in light intensity while the change is gradual between mode hops. this is,
The reflectance of the anti-reflection film 1a applied to one end surface of the optical amplifier 1 is
This occurs because the reflectance of 0% cannot be obtained completely. As a result, with the light output characteristics shown in FIG.
The output value from the arithmetic processing circuit 45 is feedback-controlled to the phase adjustment means 46 so that the wavelength ratio is changed so that the intensity ratio between the light intensity of the zero-order diffracted light L0 and the light intensity of the zero-order diffracted light L0 becomes constant.
A wavelength tunable without mode hop can be obtained.

<Second Embodiment> FIG. 3 is a block diagram showing a configuration of a phase adjusting means of an external resonator type wavelength tunable light source as a second embodiment to which the present invention is applied. In FIG. 3, the same portions as those in FIGS. 1 and 2 described above are denoted by the same reference numerals, and description thereof will be omitted. In the external resonator type wavelength tunable light source according to the second embodiment, the phase adjusting means 46 of the reflector 42 includes the second rotation control drive circuit 56, the second rotation control means 57, and the second rotation mechanism. 58
It is composed of That is, the reflector 42 is arranged on a second rotating mechanism 58 as the phase adjusting means 46, and further arranged on the first rotating mechanism 49 as the wavelength variable means 47 together with the second rotating mechanism 58. ing. The wavelength variable means 47 includes a first rotation mechanism 49 for varying the angle of the reflector 42 and changing the resonator length, a first rotation control means 51, and a first rotation control drive circuit 52.

The first rotation mechanism 49 is configured to rotate about the fulcrum 53 by driving the first rotation control means 51 under the control of the first rotation control drive circuit 52. The change of the angle of the reflector 42 and the change of the resonator length are simultaneously performed to change the Bragg wavelength λB and the resonance wavelength λFP. The second rotation mechanism 58 is configured to change the angle by driving the second rotation control means 57 based on a signal from the second rotation control drive circuit 56. However, the change in the angle of the reflector 42 changed by the phase adjusting means 46 is extremely small compared to the change in the angle changed by the wavelength variable means 47. In the configuration of the second embodiment,
This means that the Bragg wavelength λB is minutely controlled with respect to the change in the resonator length λFP by the wavelength variable means 47.

As described above, according to the external resonator type wavelength tunable light source of the second embodiment, the means for determining the wavelength deviation between the Bragg wavelength λ B and the resonance wavelength λ FP is the same as in the first embodiment. However, the method of the control means is different. As a result, similarly to the first embodiment, the arithmetic processing circuit 4
5 so that the output value from the control unit 5 is constant.
When the wavelength is tuned by performing feedback control on the wavelength, it is possible to obtain a wavelength tunable without a mode hop.

<Third Embodiment> FIG. 4 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a third embodiment to which the present invention is applied. In FIG. 4, the same parts as those in FIGS. 1 to 3 described above are denoted by the same reference numerals, and description thereof will be omitted. The diffraction grating 2 is arranged on an optical axis emitted from the end surface of the optical amplifier 1 on the antireflection film 1a side. The diffraction grating 2 functions as a wavelength-selective reflector, and converts most (reflectance) of the light having the wavelength represented by the above-described equation (1) out of the parallel light incident from the optical amplifier 1 at the incident angle θ. The first-order diffracted light L1 is reflected on the same optical path as the incident optical path, is fed back to the optical amplifier 1, and oscillates.
It should be noted that the light of the wavelength not shown by the expression (1) is reflected on an optical path different from the incident optical path, and therefore does not return to the optical amplifier 1. Further, the diffraction grating 2 is arranged on a rotation mechanism 49 as a wavelength varying means 47, and further arranged on a parallel movement mechanism 48 as a phase adjustment means 46 together with the rotation mechanism 49.

The wavelength variable means 47 comprises a rotation mechanism 49 for varying the angle of the diffraction grating 2, a rotation control means 51, and a rotation control drive circuit 52. The rotation mechanism 49 is configured to rotate by the rotation of the rotation control means 51 under the control of the rotation control drive circuit 52, and with the rotation, changes the angle of the diffraction grating 2 to change the black wavelength λB. The phase adjusting means 46 includes a parallel movement mechanism 48 for changing the diffraction grating 2 in the optical axis direction, a parallel movement control means 54, and a parallel movement drive circuit 55. Parallel movement mechanism 4
Numeral 8 is configured to move in parallel by driving the parallel movement control means 54 based on a signal from the parallel movement drive circuit 55. With the parallel movement, the resonator length changes up to the diffraction grating 2, and the resonance wavelength λFP To change. The translation drive circuit 55 drives the translation control means 54 to control the position of the diffraction grating 2 so that the value of the control signal input from the arithmetic processing circuit 45 becomes a predetermined value.

As described above, according to the external resonator type wavelength tunable light source of the third embodiment, the means for determining the wavelength difference between the Bragg wavelength λ B and the resonance wavelength λ FP is the same as the first and second embodiments. As in the example, the optical configuration of the external resonator type wavelength tunable light source is different, and the position itself of the diffraction grating 2 is controlled in parallel. As a result, similarly to the first and second embodiments, when the wavelength is tuned by performing feedback control to the phase adjusting means 46 so that the output value from the arithmetic processing circuit 45 becomes constant, the mode hop is reduced. No tunable wavelength can be obtained.

[0041]

As described above, according to the external resonator type wavelength tunable light source according to the first aspect of the present invention, in particular, feedback control is performed so as to control the position or angle of the reflector by the phase adjusting means. Since wavelength tuning is performed, even if the operating conditions (temperature, drive current, element deterioration) change, the occurrence of mode hops when the wavelength is changed is prevented, and stable wavelength tuning is performed in the wavelength band in which the laser of the optical amplifier can oscillate. It can be performed.

According to the external resonator type wavelength tunable light source according to the fourth aspect of the present invention, in particular, since the wavelength is tunable by performing feedback control so as to control the position of the diffraction grating by the phase adjusting means. Even if the conditions (temperature, drive current, element deterioration) change, it is possible to prevent the occurrence of mode hop when the wavelength is changed, and to perform stable wavelength change in the wavelength band in which the laser of the optical amplifier can oscillate.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of the external resonator type wavelength tunable light source of FIG.

FIG. 3 is a block diagram showing a configuration of a phase adjusting means of an external resonator type wavelength tunable light source according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a third embodiment to which the present invention is applied.

FIG. 5 is an explanatory diagram for explaining laser oscillation of a wavelength variable light source.

FIG. 6 is an explanatory diagram for explaining light output characteristics when the wavelength of the variable wavelength light source is changed.

FIG. 7 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a first conventional example.

8 is a block diagram showing a configuration of an external resonator type wavelength tunable light source in which the wavelength tunable means of FIG. 7 is improved.

FIG. 9 is a block diagram showing a configuration of an external resonator type wavelength tunable light source as a second conventional example.

FIG. 10 is a block diagram showing a configuration of an external resonator type wavelength tunable light source using a control means for preventing occurrence of mode hop as a third conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical amplifier 1a Non-reflection film 2 Diffraction grating 3, 4 Lens 5 Optical isolator 6 Optical amplifier drive circuit 10 Optical base table 41 Optical splitter 42 Reflector 43 First light receiver 44 Second light receiver 45 Operation processing circuit 46 Phase adjustment means 47 Wavelength variable means 48 Parallel movement mechanism 49 Rotation mechanism (first rotation mechanism) 51 Rotation control means (First rotation control means) 52 Rotation control drive circuit (First rotation control drive circuit) 53 Rotation center 54 parallel movement means 55 parallel movement drive circuit 56 second rotation control drive circuit 57 second rotation control means 58 second rotation mechanism L output light L0 0th order diffracted light Lf forward output light Lr backward output light Ls branch light

Claims (5)

[Claims] 1. An optical amplifier having an antireflection film on one end face, a diffraction grating disposed on the optical axis of the optical amplifier on the antireflection film side, having a wavelength selecting function, A reflector arranged on the optical axis of the folded light and reflecting the first-order diffracted light back to the diffraction grating, comprising: a reflector which reflects the diffraction grating at an angle symmetric to the incident angle with respect to the diffraction grating. A first photodetector that receives the next-order diffracted light and converts it into an electric signal; and a light that branches the forward output light emitted from the other end face of the optical amplifier without the non-reflection film into an output light and a branched light A splitter; a second light receiver for receiving the split light from the optical splitter and converting the split light into an electric signal; and an arithmetic processing circuit for performing predetermined arithmetic processing on the electric signals from the first and second light receivers And phase adjusting means for adjusting the position of the reflector; and output from the arithmetic processing circuit. External resonator type tunable light source and a wavelength varying means for variably controlling the wavelength and feedback control the phase adjusting means, further comprising a, wherein to control the value to a predetermined value. 2. The phase adjusting means comprises: a translation mechanism for holding a reflector; a translation control means for controlling the position of the translation mechanism; and a translation drive circuit for driving the translation control means. The position of the reflector with respect to the reflection optical axis is controlled by outputting a control signal from a parallel movement drive circuit and driving the parallel movement control means in accordance with an electric signal from the arithmetic processing circuit. 2. The external resonator type wavelength tunable light source according to 1. 3. The phase adjusting means includes: a second rotating mechanism for holding the reflector; a second rotating control means for controlling an angle of the second rotating mechanism; and a driving means for driving the second rotating control means. A second rotation control drive circuit, comprising: outputting a control signal from the second rotation control drive circuit in response to an electric signal from the arithmetic processing circuit to drive the second rotation control means; 2. The external resonator type wavelength tunable light source according to claim 1, wherein the angle of the reflector is controlled with respect to the wavelength. 4. An external resonator comprising: an optical amplifier having one end face coated with a non-reflective film; and a diffraction grating disposed on the optical axis of the optical amplifier on the non-reflective film side and having a wavelength selecting function. A wavelength-variable light source, comprising: a first photodetector for receiving zero-order diffracted light reflected at an angle symmetrical with respect to an incident angle with respect to a diffraction grating and converting the diffracted light into an electric signal; An optical splitter that splits forward output light emitted from the other end face into an output light and a split light, a second light receiver that receives the split light from the optical splitter and converts the split light into an electric signal, An arithmetic processing circuit for performing predetermined arithmetic processing on the electric signals from both the first and second light receivers; a phase adjusting means for adjusting the position of the diffraction grating; and controlling an output value from the arithmetic processing circuit to a predetermined value. To variably control the wavelength by feedback control to the phase adjustment means. External resonator type tunable light source, characterized in that, with a wavelength varying mechanism, the. 5. A position adjusting means comprising: a parallel movement mechanism for holding a diffraction grating; a parallel movement control means for controlling the position of the parallel movement mechanism; and a parallel movement drive circuit for driving the parallel movement control means. And controlling the position of the diffraction grating with respect to the emission optical axis by outputting a control signal from the parallel movement drive circuit and driving the parallel movement control means in accordance with an electric signal from the arithmetic processing circuit. 4. An external resonator type wavelength tunable light source according to item 4.