JP3309903B2 - Mode-locked semiconductor laser and driving method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to optical communication, optical measurement,
The present invention relates to a monolithic integrated semiconductor laser used for optical information processing and a driving method thereof.

[0002]

2. Description of the Related Art In recent years, demands for increasing the capacity of optical communication technology have been increasing, and devices and optical communication systems for responding to the demand have been actively studied. As optical communication systems with large capacity, mainly two types, a wavelength multiplexing system and a time division multiplexing system, are considered. By using these together, it is considered that a realistic large capacity communication system will be constructed. . As the time division multiplexing method, an optical time division multiplexing method using pulsed light has attracted attention. In this optical time division multiplex system,
After modulating the pulse light of picosecond order by an optical modulator for each channel, and multiplexing it with a certain delay, an ultra-high-speed time-multiplexed optical signal train exceeding 100 Gbit / s is obtained. Can be

A mode-locked semiconductor laser is promising as a light source for optical communication such as an optical time division multiplexing method because it can generate ultrafast pulses in the order of picoseconds relatively easily and stably.

[0004] The main characteristics required for a light source for optical communication are the operating wavelength and frequency of the light source. The operating wavelength needs to be precisely controlled around 1.55 μm. Further, the generated pulse needs to be a pulse at the limit of the Fourier transform, and the oscillation wavelength width also needs to be controlled. In order to satisfy these requirements, there has been reported a mode-locked semiconductor laser in which a distributed reflector using a diffraction grating is integrated.

On the other hand, since the frequency is strictly defined by the communication system, it is necessary to match the operating frequency of the mode-locked semiconductor laser. Since the repetition frequency of the optical pulse of the mode-locked semiconductor laser is given by the reciprocal of the round trip time of the optical pulse in the laser resonator, in order to set the frequency accurately, the dimensions of the optical resonator length must be precisely manufactured. There is a need. The accuracy required for the resonator length is determined as follows.

That is, in driving a mode-locked semiconductor laser, a forced mode in which a gain or a loss is modulated by externally applying a sine wave voltage corresponding to the system frequency in order to fix the clock frequency of the system. Synchronous or hybrid mode-locking techniques are used. At this time, the frequency determined by the resonator length is 0.1% of the modulation frequency.
With a slight deviation, the mode synchronization frequency can be synchronized with an external modulation frequency. Therefore, the manufacturing accuracy of the resonator length is required to be 0.1%.

When the manufacturing accuracy of 0.1% is applied to an actual device, in the case of a mode-locked semiconductor laser operating at 10 GHz, the resonator length is about 4 mm, and the manufacturing accuracy is 4 μm.
Becomes However, it is generally difficult to fabricate the resonator length with high accuracy using a semiconductor laser. This is due to the fact that the laser cavity is formed by cleavage,
It is very difficult to form a resonator with high accuracy and high reproducibility.

In order to solve the problem of the cleavage accuracy, two methods have been tried. The first method is to change the optical length by changing the refractive index of the waveguide from the outside to change the frequency, and the second method is to change the depth at which light enters the portion of the distributed reflector from the outside. In this method, the frequency is controlled and the frequency is made variable using the change in the length. Examples disclosed as these methods will be described. FIG.
FIG. 13 is a diagram showing a conventional example of a mode-locked semiconductor laser having a function of correcting a frequency by changing a refractive index, which is the first method.
"Short pulse gene" published in Quantum Electronics, Volume 28, pages 2186 to 2202.
ration using multisegment mode-locked semiconducto
r lasers "and JP-A-4-2190.

According to FIG. 5, this mode-locked semiconductor laser has a structure in which a plurality of regions are provided in one waveguide structure 50 including an active layer, and each region is driven by an individual electrode. 51, a gain region 52, a gain modulation region 53, passive waveguide regions 54 and 55, and a distributed reflector 56
It is composed of A diffraction grating 57 is formed on the waveguide portion of the distributed reflection mirror 56.

The operation of the mode-locked semiconductor laser is such that a current is injected into the gain region 52, a passive mode-locking operation is performed by reverse-biasing the saturable absorption region 51, and an external signal is modulated by modulating the gain modulation region 53 from outside. A hybrid mode-locking operation for synchronizing the mode-locking frequency with the source can be performed. The distributed reflector 56 has wavelength selectivity, and a mode-locking operation occurs at a Bragg wavelength determined by the period of the diffraction grating 57. The mode locking frequency is determined by the length of the resonator. By injecting carriers into the passive waveguide regions 54 and 55, the frequency is adjusted by changing the refractive index and the function of adjusting the phase of the pulse is provided. ing.

The second method is an example in which a mode-locked semiconductor laser having substantially the same structure as that of FIG. 5 is used, and the area of the distributed reflector is divided into a plurality of parts so that each can be individually driven electrically. This is disclosed in JP-A-8-340152. In this document, in order to change the number of repetitions of mode-locking, the distributed reflection mirror is divided, and the relationship between the refractive index and the absorption coefficient between the regions is changed to change the penetration length of the incident light. It is characterized by: As a specific method, each of the divided distributed mirror regions is divided into a region into which current is injected and a region into which current is not injected, and is made to transmit light by current injection. The only advantage is that the light penetration length is increased. As a result, by sequentially injecting current from the inside of the resonator, the mode-locking frequency gradually changes in units of the length of the region.

[0012]

However, the method of making the mode locking frequency variable as described above has the following problems. In the first method of changing the operating frequency of the mode-locked semiconductor laser by changing the refractive index of the passive waveguide, the change in the refractive index that can be controlled from the outside is at most 1% or less, and the length of the passive waveguide is limited. Therefore, even if the optical length is changed in that region, the amount of change in the entire length of the resonator is small, so the frequency variable range is narrow, and sufficient frequency adjustment to compensate for the cleavage accuracy is performed. Difficult to do.

Also, as in the conventional example shown in FIG.
Although it is necessary to provide a passive waveguide region for changing the refractive index separately from a gain region for generating mode locking, etc., in a device having a short resonator length and a high repetition frequency of several tens of GHz or more, the passive waveguide is not provided. This technique cannot be used because there is not enough room to insert.

In the method of dividing the second distributed reflector into a plurality of parts and injecting current, the following problems occur.

In Japanese Patent Application Laid-Open No. 8-340152, the main purpose of this method is to make the mode-locking repetition frequency changeable. However, in Japanese Patent Application Laid-Open No. 4-2190, which discloses the same structure, the main purpose of such a driving method is to change the pulse width of mode locking by changing the transmission spectrum width of the distributed reflector. . That is, changing the mode locking frequency also changes the pulse width. This is because the reflection spectrum band of the distributed reflector depends on the length of the distributed reflector.

It is well known that when current is injected into a semiconductor, the refractive index changes due to the plasma effect.
This is used for a wavelength tunable semiconductor laser in which the refractive index is changed by injecting current into the distributed reflector and the center wavelength (Bragg wavelength) of the distributed reflector is changed. The method of making the mode locking frequency variable by the method of making the transmission state involves a large wavelength change.

As described above, in the conventional example, when the driving for changing the mode locking frequency is performed, the pulse width and the wavelength are changed at the same time.

To summarize the above problems, the frequency of the mode-locked semiconductor laser is determined by the length of the resonator, and it is difficult to satisfy the frequency accuracy required for communication with the current accuracy of the device fabrication process. In the conventional frequency variable method which has been tried to solve this, there is a limit to the element length that can be realized in the first method, and in the second method, the pulse width and operating wavelength are simultaneously changed. Therefore, even if the frequencies are adjusted, it is difficult to simultaneously satisfy the desired wavelength and pulse width.

An object of the present invention is to eliminate the drawbacks of the conventional mode-locked semiconductor laser and to reduce the frequency from several GHz to several
A structure of a mode-locked semiconductor laser capable of compensating for a frequency shift of the mode-locked semiconductor laser caused by manufacturing accuracy at any frequency of 0 GHz or more and keeping a wavelength and a pulse width within a certain range in the frequency variable operation. And a driving method.

[0020]

A mode-locked semiconductor laser according to the present invention has a structure in which at least an optical gain region, a saturable absorption region, and a distributed reflection region having a diffraction grating are coupled by an optical waveguide. It is characterized in that the absorption loss of the layer is variable by applying a voltage from outside.

More specifically, the composition and structure of the active layer in the distributed reflection region are such that the transition wavelength is on the short wavelength side with respect to the transition wavelength between the optical amplification region and the saturable absorption region, ie, the emission wavelength at which mode-locking operation occurs. And it can be shifted to a mode-locked operation wavelength by applying an external voltage. Further, the method of driving the mode-locked semiconductor laser causes a mode-locking operation by applying a current injection to the optical amplification region and applying a reverse bias voltage to the saturable absorption region.
It is characterized in that a forward bias or a reverse bias lower than the level at which current injection occurs is applied.

According to the present invention, a mode-locked semiconductor laser in which the effective length of a distributed reflector is variable by externally controlling the absorption loss of the distributed reflector using a diffraction grating. Can be changed over a wide range. Thus, a mode-locked semiconductor laser having a frequency exactly matching the clock frequency of the optical communication system is provided.

[0023]

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the overall configuration of a mode-locked semiconductor laser according to the present invention. According to the figure, the semiconductor laser device has a distributed reflector on one side and a cleavage plane on the other. The mode-locked semiconductor laser according to the present invention has at least a gain region 12, a saturable absorption region 11, and a distributed reflection mirror 14.
In addition, an absorption light modulation region 13 is integrated as a region for performing a hybrid mode-locking operation for modulating optical loss or optical gain by externally inputting a system frequency signal. I have. Saturable absorption region 11 instead of absorption light modulation region 13
Alternatively, the gain area 12 can be modulated.

The mode-locked semiconductor laser according to the present invention has a structure in which an active layer 21 is formed on an n-InP substrate 10 to have a width of 1.5 μm, and is embedded with a p-InP cladding layer 22. The diffraction grating 20 is formed on the substrate 10 in advance in the distributed reflector 14, and is embedded by crystal growth simultaneously with other regions.

When considering the resonator length of an element in which a distributed reflector is integrated, an effective length (depth of light penetration into the distributed reflector) is defined as an effective length of the distributed reflector. Using the length, the resonator length can be considered in the same manner as a normal cleavage plane reflecting mirror. For the definition of effective length, see "IEEE Journal of Quantum Electronics" published in 1983.
The article “1.5-1.6 μm GaInAsP / InP dynamic-s” published on pages 1042 to 1051 of volume QE-19
ingle-mode (DSM) lasers with distributed Bragg ref
The effective length of the distributed reflector is determined by the coupling constant κ of the diffraction grating in an ideal case where there is no loss. In this case, the effective length is approximately expressed as 1 / (2κ + α) by the coupling constant κ and the absorption loss α. The coupling constant is determined by the difference in refractive index between the irregularities formed in the waveguide, and is usually about 30 cm ^-1 .

FIG. 2 shows the result of calculating the relationship between the absorption loss of the distributed reflector and the effective length. The coupling constant is 30 cm ^-1 . Thus, it can be seen that the effective length of the distributed reflector changes by several tens of μm depending on the value of the absorption loss. Since the pulse repetition frequency of the mode-locked semiconductor laser changes by an amount corresponding to the change in the effective length of the distributed mirror, the mode-locking frequency can be made variable if the absorption loss of the distributed mirror can be intentionally changed. If the effective length changes by several tens of μm, it is possible to give a length change of 1% or more for a resonator length of about 4 mm of a mode-locked semiconductor laser operating at 10 GHz, and an error generated in the resonator length due to normal cleavage. Is an amount sufficient to correct.

As a conventional example, Japanese Patent Application Laid-Open No. 8-340 described above.
No. 152 discloses a structure in which the active layer of the distributed mirror has a composition having a transition wavelength substantially equal to the wavelength of the mode-locking operation, gives a large absorption loss when no bias is applied, and becomes transparent by current injection. The way to change it. Although it is possible to change the absorption loss by this method as well, since the refractive index changes due to the current injection, the disadvantage that the wavelength changes simultaneously with the frequency occurs as described above as a problem. .

The structure of the present invention differs from the conventional example in that the absorption loss is changed by a method not depending on the current injection. Examples will be described below. The active layer 21 in the portion of the distributed reflector 14 is
Distortion MQW (Multi Quat) having an absorption edge at a wavelength of 1.48 μm
ntum Well) structure, and the absorption edge is 1.55 μm, which is the longer wavelength side, that is, the mode-locking operation wavelength, due to the quantum confined Stark effect when a reverse bias is applied.
, And the absorption loss for the mode-locked pulsed light increases significantly. Specifically, the absorption is substantially zero at the time of forward bias of about 0.7 V at which the current injection starts, and the absorption coefficient becomes about 200 cm ^-1 at the reverse bias of about 2 V. It should be noted that in addition to the absorption loss in the active layer, a constant absorption loss of about 10 cm ^-1 exists due to free carrier absorption in the clad layer.

The portion of the gain region 12 and the saturable absorption region 11 has an energy gap wavelength of the active layer of 1.55 μm.
The absorption light modulation region 13 has the same structure as the distributed reflection mirror 14. As a method of collectively forming these regions having different energy gaps, a selective growth technique using a gas phase growth method was used. The specific growth method is disclosed in detail in JP-A-3-19411 and JP-A-3-216027.

With this structure, it is possible to change the absorption loss of the distributed mirror by applying a voltage from the outside and control the effective length to tune the mode locking frequency. Here, the refractive index change due to the application of a voltage will be described. The refractive index changes with a change in absorption due to the well-known Klamath-Kronig relationship. However, as will be described later, the amount of change in the refractive index is small within a range in which the absorption loss is changed for the purpose of frequency correction, which is an object of the present invention. It is considered that there is no large wavelength change.

[0032]

Next, an embodiment of a method of driving a mode-locked semiconductor laser according to the present invention will be described. Here, the SDH frequency (9.95328GH) which is one of the communication frequency standards is used.
The result of frequency tuning of the mode-locked semiconductor laser in z) will be described. The total length of the resonator when the device was manufactured by manual cleavage was 4240 μm. The target value of the pulse width of the distributed reflector was 5 to 6 ps, and the length of the distributed reflector was 250 μm.

FIG. 3 shows that the injection current into the gain region is 95 m.
A, the results of measuring the change in the mode locking frequency by fixing the bias voltage of the saturable absorption region (SA: Saturable Absorption) to -1.0 V and changing the voltage applied to the distributed reflector. Since the absorption loss, which is the principle of frequency tuning, decreases as the reverse bias is changed to the forward bias, the effective length of the distributed reflector becomes longer and the frequency is expected to shift to a lower frequency during this time.
As shown in the figure, the frequency monotonously decreased as the frequency was biased in the forward direction, and a result similar to the principle of frequency tuning was obtained.

FIG. 3 shows the oscillation wavelength measured at the same time. During this period, the wavelength is changed only by 0.05 nm, and is hardly affected by frequency tuning.

A change in frequency of 0.5% is obtained, and 0.2% is obtained.
At a bias voltage of 2 V, the SDH frequency (9.953)
28 GHz). As a result, it was confirmed that errors in the cavity length due to normal cleavage could be sufficiently corrected.

FIG. 4 shows the result of observing the frequency spectrum at the time of SDH frequency operation. In the state where passive mode locking is performed, a sine wave voltage of a synthesizer is applied to the EA modulation region and 9.95328 GHz is used in hybrid mode locking operation.
Shows a locked state. The pulse width at this time was 6.2 ps (sech ² type) almost as designed, the wavelength was 1.552 μm, and the time-frequency width product was 0.39, a value close to the Fourier transform limit.

[0037]

As described above, according to the present invention, the frequency of the mode-locked semiconductor laser can be changed in a wide range. This makes it possible to provide a mode-locked semiconductor laser having a frequency exactly matching the clock frequency of the optical communication system.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a mode-locked semiconductor laser of the present invention.

FIG. 2 is a diagram showing a result of calculating an effective length of a distributed reflector according to the principle of the present invention.

FIG. 3 is an embodiment of a frequency variable operation of the mode-locked semiconductor laser according to the present invention.

FIG. 4 is a frequency spectrum of an output pulse in a mode locking operation.

FIG. 5 is a diagram showing a conventional example of a mode-locked semiconductor laser having a frequency adjustment function.

[Explanation of symbols]

 Reference Signs List 10 n-InP substrate 11 saturable absorption region 12 gain region 13 absorption light modulation region 14 distributed reflector 20 diffraction grating 21 active layer 22 p-InP cladding layer 30 output pulse train 50 waveguide structure 51 saturable absorption region 52 gain region 53 Gain modulation region 54, 55 Passive waveguide 56 Distributed reflector 57 Diffraction grating

Continuation of the front page (56) References JP-A-6-152049 (JP, A) JP-A-4-2190 (JP, A) JP-A-8-340152 (JP, A) Semiconductor Laser Conference, 1996. , 15th IEEE International, p. 55-56 Applied Physics Letters, 64 [15], p. 1917-1919 IEEE Journal of Quantum Electronics, 32 [11], p. 1965-1975 Electronics Letters, 29 [11], p. 1013-1015 IEEE Journal of Quantum Electronics, 28 [10], p. 2186-2202 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (7)

(57) [Claims] 1. A mode-locked semiconductor laser in which at least an optical amplification region, a saturable absorption region, and a distributed reflection region having a diffraction grating are coupled by an optical waveguide, wherein the absorption loss of the active layer in the distributed reflection region is reduced. A mode-locked semiconductor laser comprising control means for controlling by applying a voltage from outside. 2. The control unit according to claim 1, wherein the control unit controls an effective length of the distributed reflection mirror in the distributed reflection area by applying an external voltage.
The mode-locked semiconductor laser as described. 3. The mode-locked semiconductor laser according to claim 2, wherein the control of the effective length of the distributed reflector controls the repetition frequency of an optical pulse. 4. The active layer of the distributed reflection region has a transition wavelength on a short wavelength side with respect to a transition wavelength which is an emission wavelength at which a mode-locking operation of the optical amplification region and the saturable absorption region occurs. 2. The mode-locked semiconductor laser according to claim 1, wherein the wavelength can be shifted to a mode-locked operation wavelength by applying a voltage from the semiconductor laser. 5. A method for driving a mode-locked semiconductor laser in which at least an optical amplification region, a saturable absorption region, and a distributed reflection region having a diffraction grating are coupled by an optical waveguide, wherein a current is supplied to the optical amplification region. Injecting and applying a reverse bias voltage to the saturable absorption region to generate a mode-locked operation; and applying a forward bias or a reverse bias equal to or lower than a level at which current injection occurs to the distributed reflection region. Controlling the absorption coefficient of the distributed reflection region by applying the bias. 6. The driving of the mode-locked semiconductor laser according to claim 5, wherein the step of controlling the absorption coefficient of the distributed reflection region is a step of controlling an effective length of the distributed reflection mirror in the distributed reflection region by an applied voltage value. Method. 7. The method for driving a mode-locked semiconductor laser according to claim 6, wherein the step of controlling the effective length of the distributed reflection region is a step of controlling the repetition frequency of the light pulse by an applied voltage value.