JP2011253977A - Dbr laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DBR laser having a narrow oscillation line width, capable of stably performing a single mode oscillation operation even under a high output state.SOLUTION: A DBR laser of the present invention includes an active region having an active waveguide layer, and a DBR mirror region having a passive waveguide layer provided adjacent to the active region on an extension of the active waveguide layer. A plurality of modulation regions to modulate a refraction factor of light or a gain of a mode are provided on a region away from the light output side end of a region in an effective cavity length, by a distance of the effective cavity length, which is the sum of an immersion length of light to the DBR mirror region and an active region length, divided by the integer, within the active region.

Description

  The present invention relates to a single-mode semiconductor laser having a narrow oscillation line width suitable for use in optical fiber communication, particularly coherent optical communication.

  Currently, many optical fiber communication systems employ an intensity modulation / direct detection method in which laser beam intensity modulation is performed on the transmission side and direct detection is performed on the reception side. Although this method requires a simple configuration of the transmission / reception device, there is room for improvement in terms of reception sensitivity and frequency utilization efficiency. Therefore, in recent years, the adoption of optical communication systems using phase modulation such as DPSK (Differential Phase Shift Keying) suitable for increasing the capacity of the system and increasing the transmission speed has begun. Accordingly, coherent optical communication technology, which is a communication method that uses light frequency and phase information, has been attracting attention again.

  In coherent optical communication, reception sensitivity and frequency utilization efficiency can be increased by adopting a heterodyne or homodyne detection method, and therefore, much research and development has been made mainly in the 1980s. A device indispensable for the practical use of this system is a semiconductor laser light source that is used as a local oscillation light source on the receiving side and can stably oscillate in a single longitudinal mode and has a narrow oscillation line width of 1 MHz or less.

  As a conventional single mode semiconductor laser, a distributed feedback type (DFB) semiconductor laser in which a diffraction grating is provided in the vicinity of an active layer or a passive waveguide layer and oscillates at a specific wavelength corresponding to the period thereof, or a distribution is provided. A typical example is a Bragg reflection type (DBR: Distributed Bragg Reflector) semiconductor laser. Since these lasers can easily obtain a side mode suppression ratio (SMSR) of 30 dB or more, they are expected as single longitudinal mode light sources for coherent communication.

  Such a conventional single mode semiconductor laser can perform stable single mode oscillation at a low current injection level of several tens of mA or less. The oscillation line width Δf of the single mode semiconductor laser is

(See Non-Patent Document 1). Here, R is the spontaneous emission rate, I is the optical output, and α is an α parameter that represents the ratio of the change amount of the real part and the imaginary part of the refractive index to the change amount of the carrier density. From this equation, the oscillation line width of the semiconductor laser is ideally reduced in inverse proportion to the optical output.

Charles H. Henry, "Theory of the Linewidth of Semiconductor Lasers", IEEE Journal of Quantum Electronics, (USA), Vol.18, No.2, 1982, pp .259-264.

  However, in the conventional λ / 4 shift DFB laser, the photon density increases in the phase shift region, and the carrier density decreases. This tendency becomes more prominent when the current injection level into the active layer becomes higher. As a result, a carrier density distribution (that is, a refractive index distribution) is formed in the longitudinal direction of the resonator. There is a problem that the oscillation mode becomes unstable due to the influence of the spatial hole burning effect.

  Similarly, in the DBR laser, when the current injection level into the active layer increases, the gain peak position of the active layer shifts or the gain spectrum shape changes due to the temperature rise of the active region, etc., and oscillates. The wavelength positions of the longitudinal mode and the DBR reflection peak are shifted. As a result, contention with the adjacent longitudinal mode occurs and the SMSR deteriorates, so that the oscillation line width increases. Therefore, a complicated and high-cost feedback control mechanism that adjusts the phase current while monitoring the change in the SMSR is required. Furthermore, when the active layer region length is increased for higher output, the longitudinal mode interval is narrowed, which makes it difficult to achieve stable single mode oscillation.

  In view of the above-described problems, an object of the present invention is to provide a DBR laser that can stably operate in a single mode even in a high output state and has a narrow oscillation line width.

  The DBR laser of the present invention is a DBR laser including an active region having an active waveguide layer and a DBR mirror region provided adjacent to the active region and having a passive waveguide layer on an extension of the active waveguide layer. The active resonator is separated from the light output side end of the effective resonator length region by an integral fraction of the effective resonator length, which is the sum of the light penetration length into the DBR mirror region and the active region length. A plurality of modulation regions for modulating the refractive index of light or the gain of a mode are provided in a region in the region.

  The DBR laser of the present invention has an optical output side end portion of an effective resonator length region by an integral fraction of an effective resonator length, which is the sum of the penetration depth of light into the DBR mirror region and the active region length. Even if the effective resonator length is long and the oscillation mode interval is narrowed by providing multiple modulation regions that modulate the refractive index or mode gain of light in the active region away from Is possible.

2 is a cross-sectional view showing a structure of a DBR laser according to Embodiment 1. FIG. 6 is a cross-sectional view showing a structure of a DBR laser according to a second embodiment. FIG. 6 is a cross-sectional view showing a structure of a DBR laser according to a third embodiment. It is a figure which shows operation | movement of the conventional DBR laser. It is a figure which shows operation | movement of the DBR laser of this invention. It is sectional drawing which shows operation | movement of the conventional (lambda) / 4 shift DFB laser. It is sectional drawing which shows the structure of the conventional DBR laser. It is a figure which shows the relationship between the oscillation line width of a conventional single mode semiconductor laser, and optical output.

<Prerequisite technology>
FIG. 6 shows a cross-sectional view of the element structure of a λ / 4 phase shift DFB laser as a single mode semiconductor laser according to the prerequisite technology of the present invention. The λ / 4 phase shift DFB laser is suitable for narrow linewidth operation among DFB lasers. 6A is a front view seen from the front light output side, and FIG. 6B is a cross-sectional view taken along line AA ′ of FIG. For simplicity, upper and lower electrodes, contact layers, and wiring are not shown.

  The λ / 4 phase shift DFB laser includes an active waveguide 7 composed of an active layer 1 and an optical confinement layer 2 disposed above and below the active layer 1. The active waveguide 7 is sandwiched between the upper cladding layer 4 and the lower cladding layer 5, and the side surfaces are covered with the current blocking layer 6. In the case of a long-wave band communication element, the cladding layers 4 and 5 and the current blocking layer 6 are generally composed of InP, the active layer 1 is composed of InGaAsP multiple quantum wells, and the optical confinement layer 2 is composed of bulk InGaAsP. The active waveguide 7 including the active layer 1 exists in the entire longitudinal direction (length L) of the element, and antireflection coating 8 is applied to both end faces thereof. In addition, a diffraction grating 9 corresponding to a desired wavelength is provided in the vicinity of the active waveguide 7, and a phase shift region 10 corresponding to a quarter wavelength in the semiconductor is further provided in the center thereof.

  When laser oscillation occurs due to current injection into the active layer 1, an equal amount of output light 11 is emitted from the front and rear end faces (the rear output light is not shown).

  Next, FIG. 7 shows an example of the configuration of the DBR laser. A front view seen from the front output side is omitted because it is the same as FIG. 6A, and FIG. 7 is a longitudinal sectional view similar to FIG. 6B. For simplicity, the upper and lower electrodes, the contact layer, and the wiring are not shown in FIG. 6, and the same reference numerals are given to the same components as those in FIG. Unlike the DFB laser, the active waveguide 7 exists only in the active region on the front output side of the device. Behind the active region, a phase adjustment region and a DBR mirror region are provided, both of which consist of a passive waveguide 15. The front end face of the element is a cleavage end face, and a non-reflective coating 8 is applied to the rear end face. A diffraction grating 9 is provided in the vicinity of the DBR mirror region to constitute a DBR reflecting mirror.

The DBR laser is basically a Fabry-Perot resonator type semiconductor laser composed of a cleaved end face and a DBR reflecting mirror, and laser oscillation occurs when current is injected into the active layer 1. Most of the guided light that has entered the DBR mirror region is reflected at a distance called the penetration length L _pen, and the combined length of the active region, the phase adjustment region, and L _pen becomes the effective resonator length L _eff . In a Fabry-Perot resonator type semiconductor laser, there are many longitudinal modes 18 with a wavelength interval Δλ = λ ² / (2 × n _eff × L _eff ), where n _eff is an effective refractive index, but it is determined by the Bragg wavelength of the diffraction grating. Only the longitudinal mode near the peak of the DBR reflection spectrum 19 is selected. The peak width and height (reflectance) and L _pen of the DBR reflection spectrum 19 are the product of the coupling coefficient κ of the diffraction grating and the DBR mirror region length. If the reflectivity of the DBR reflector is high, most of the output light 11 is emitted from the front, but the peak width increases as the reflectivity is increased, so it is necessary to set an appropriate value. By controlling the injection current to the phase adjustment region and performing phase adjustment, one longitudinal mode can be selectively oscillated, and an oscillation line width comparable to that of the DFB laser can be obtained. In addition, when current is injected into the DBR mirror region, the refractive index is lowered due to the band filling effect. By combining with the adjustment of the phase adjustment current, the oscillation wavelength can be shifted to the short wavelength side by several nm at the maximum. is there.

  In such a single mode semiconductor laser, it is expected from the equation (1) that the oscillation line width of the semiconductor laser is ideally narrowed in inverse proportion to the optical output.

  However, in the conventional λ / 4 shift DFB laser, as shown in FIG. 6, the photon density increases in the phase shift region, and the carrier density decreases. This tendency becomes more prominent when the current injection level into the active layer becomes higher. As a result, a carrier density distribution (that is, a refractive index distribution) is formed in the longitudinal direction of the resonator. The oscillation mode becomes unstable due to the influence of the spatial hole burning effect.

  Even in the DBR laser, when the active layer region length is increased to increase the output, the longitudinal mode interval is narrowed, and thus there is a problem that stable single mode oscillation becomes difficult.

  Due to the above effects, in the conventional single mode semiconductor laser, as shown in FIG. 7, the optical output level increases and the oscillation line width becomes narrower. However, when the output level exceeds a certain output level, the oscillation line width tends to increase again. It was difficult to obtain a minimum line width of 1 MHz or less.

  Therefore, in the present invention, as shown below, by forming a modulation region in the active region in a DBR laser, single mode oscillation can be stably performed even at high output, and the oscillation line width is narrowed.

(Embodiment 1)
<Configuration>
FIG. 1 schematically shows the element structure of the DBR laser according to the first embodiment. FIG. 1A is a front view seen from the output side in front of the element, and FIG. 1B is a cross-sectional view taken along line AA ′ of FIG. For simplicity, the upper and lower electrodes are not shown. As a material system, the following description will be given in the case where an InGaAsP waveguide layer is formed on an InP substrate, which is generally used in long wave optical communication.

  The DBR laser includes an active region and a DBR mirror region behind the active region. The DBR laser has an active waveguide layer 7 made of InGaAsP in the active region, an upper cladding layer 4 made of p-InP and an lower cladding layer 5 made of n-InP that sandwich the active waveguide layer 7 from above and below, and an active waveguide. A current blocking layer 6 having a deposited structure of p-InP and n-InP covering the side surface of the layer 7 is provided. The active waveguide layer 7 includes an active layer 1 and an optical confinement layer 2 disposed above and below the active layer 1.

  The DBR laser includes a passive waveguide layer 15 disposed on the extension of the active waveguide layer and a diffraction grating 9 disposed near the upper portion of the passive waveguide layer 15 in the DBR mirror region. The configuration around the passive waveguide layer 15 is the same as that of the active region. The upper and lower layers of the passive waveguide layer 15 are sandwiched between the upper cladding layer 4 and the lower cladding layer 5, and the current blocking layer 6 covers the side surfaces.

An end face in front of the element that outputs oscillation light is a cleavage end face, and a non-reflective coating 8 is applied to the end face of the rear DBR mirror region. A Fabry-Perot resonator is constituted by the DBR mirror formed by the diffraction grating 9 and the cleavage end face, and the combined length of penetrating light L _pen into the DBR mirror region and the active region is the effective resonator length L _eff . Become.

  Although laser oscillation occurs when current is injected into the active layer 1, the coupling coefficient κ of the diffraction grating 9 and the DBR mirror are set so that the reflectivity of the DBR mirror becomes 90% or more in order to increase the intensity of output light emitted from the front. The region length is chosen. At this time, the peak width of the DBR reflection spectrum 19 widens up to about 3 to 5 nm.

On the other hand, in order to increase the maximum output of the DBR laser, it is effective to increase the volume of the active layer while maintaining the waveguide mode. Specifically, it is necessary to increase the active region length to several hundred μm or more. At this time, L _eff also becomes long, so that the interval Δλ of the longitudinal mode 18 also becomes narrow. Therefore, as compared with the conventional DBR laser shown in FIG. 7, there are more longitudinal modes inside the peak width of the reflection spectrum 19.

Therefore, the DBR laser according to the present embodiment is made of InGaAsP and close to the active waveguide layer 7 via the InP layer at a modulation position that is L _eff / N (N: integer) away from the cleaved end face in the active region. A modulation region 22 for modulating the refractive index is provided. Alternatively, the modulation region may be a region that modulates the gain of the longitudinal mode, but will be described below as the refractive index modulation region 22 that modulates the refractive index.

  The refractive index modulation region 22 can be formed by a manufacturing process in which an InGaAsP layer is subjected to patterning by photolithography and dry etching, and then is embedded and grown with InP. The length of each refractive index modulation region 22 may be selected to be 10 μm or less, for example, 5 μm.

That is, the DBR laser of the present embodiment includes an active region having an active waveguide layer 7 and a DBR mirror region provided adjacent to the active region and having a passive waveguide layer 15 on the extension of the active waveguide layer 7. The light in the region of the effective resonator length is an integral fraction of the effective resonator length L _eff which is the sum of the light penetration length L _pen and the active region length into the DBR mirror region. By providing a plurality of modulation regions 22 for modulating the refractive index or mode gain of light in a region in the active region away from the end on the output side, single mode oscillation is stably performed up to a high light output, and a narrow oscillation line Get the width.

<Operation>
Next, the vertical mode selection operation will be described. 4 and 5 are explanatory diagrams of the modulation effect of the oscillation mode by the refractive index modulation region 22. For example, if the effective resonator length is L _eff = 600 μm, the longitudinal mode interval in the long wave band is about 0.5 nm, as shown in FIG. If the peak width of the DBR reflection spectrum 19 is 3.5 nm, a total of seven of the longitudinal modes shown in FIG. 4A are selected by filtering (FIG. 4B).

Here, when the refractive index modulation region 22 is provided in the active region, slight reflection occurs in the waveguide mode. Therefore, the gain of the longitudinal mode at the corresponding wavelength position is changed to other longitudinal modes while the reflection in the resonator is repeated. It is larger than the mode and undergoes mode gain modulation. For example, the longitudinal mode modulated by the refractive index modulation region 22 whose center position is L _eff / 5 from the cleavage end face is as shown in FIG. 5A, and the mode gain is 5 times the original longitudinal mode interval. Modulated. Similarly, the longitudinal mode modulated by the refractive index modulation region 22 whose center position is located at L _eff / 2 from the cleavage end face is as shown in FIG. 5B, and the mode gain is twice the original longitudinal mode interval. Is modulated. Therefore, when two refractive index modulation regions 22 having these center positions L _eff / 5 and L _eff / 2 are combined, these effects are added to the reference wavelength, as shown in FIG. A simple modulation spectrum can be obtained. By combining the longitudinal mode selection mechanism based on the action of the refractive index modulation region 22 with the above-described DBR filter, only a specific longitudinal mode can be selected, and phase adjustment current control is not performed as in the conventional DBR laser. It is possible to oscillate a single longitudinal mode even during output.

  As described above, according to the present invention, single mode oscillation can be stably performed up to a high optical output, and a narrow oscillation line width can be obtained as compared with the conventional λ / 4 shift DFB laser and DBR laser.

<Effect>
According to the DBR laser of the present embodiment, the following effects can be obtained as described above. That is, the DBR laser of the first embodiment includes an active region having an active waveguide layer 7 and a DBR mirror region that is provided adjacent to the active region and has a passive waveguide layer 15 on the extension of the active waveguide layer 7. The light in the region of the effective resonator length is an integral fraction of the effective resonator length L _eff which is the sum of the light penetration length L _pen and the active region length into the DBR mirror region. A plurality of modulation regions 22 that modulate the refractive index of light or the gain of the mode are provided in a region in the active region far from the output side end. As a result, even when the effective resonator length L _eff is increased to achieve high output and the longitudinal mode interval is narrowed, single mode oscillation can be stably performed and a narrow oscillation line width can be obtained.

  The DBR mirror region is provided only on one end side of the active region, and the other end of the active region is a cleavage end surface. Even with such a configuration, it is possible to stably oscillate a single mode until a high output and obtain a narrow oscillation line width.

(Embodiment 2)
In the first embodiment, the DBR mirror region is provided only behind the active region. However, in the second embodiment, the DBR mirror region is provided in front of and behind the active region as shown in FIG. Since the other configuration is the same as that of the first embodiment, the description thereof is omitted. The same components as those in the first embodiment shown in FIG.

In this case, the effective resonator length L _eff is the sum of the penetration length of the front DBR, the active region length, and the penetration length of the rear DBR. Therefore, as shown in FIG. 2, the refractive index modulation region 22 is provided at the center positions L _eff / 5 and L _eff / 2, respectively, from the front end of the effective resonator region, so that the same as in the first embodiment. In addition, it is possible to oscillate a single longitudinal mode until high output, and a narrow oscillation line width can be obtained.

<Effect>
According to the present embodiment, the following effects can be obtained. That is, the DBR mirror region is formed adjacent to both ends of the active region. Even with such a configuration, it is possible to oscillate a single longitudinal mode until high output, and a narrow oscillation line width can be obtained.

(Embodiment 3)
FIG. 3 shows the configuration of the DBR laser according to the present embodiment. The DBR laser of the third embodiment is provided with a thin film heater 23 for heating the DBR mirror region in addition to the configuration of the first embodiment. Since the configuration other than this is the same as that of the first embodiment, the description thereof is omitted.

By energizing and heating the thin film heater 23, the peak wavelength of the DBR reflection spectrum 19 can be variably controlled without injecting current into the DBR mirror region. Since the loss of the DBR mirror region does not increase and the peak reflectance and L _{eff of} the DBR mirror region do not change, it becomes possible to maintain single mode oscillation more stably.

<Effect>
According to the present embodiment, the following effects can be obtained. That is, the DBR laser of the third embodiment includes the thin film heater 23 for heating the DBR mirror region in addition to the configuration of the first embodiment, so that the peak of the DBR reflection spectrum 19 can be obtained without injecting current into the DBR mirror region. The wavelength can be variably controlled. For this reason, the loss of the DBR mirror region does not increase, and the peak reflectance and L _{eff of} the DBR mirror region do not change, so that it is possible to maintain single mode oscillation more stably.

  1 active layer, 2 optical confinement layer, 4 upper cladding layer, 5 lower cladding layer, 6 current blocking layer, 8 anti-reflection coating, 9 diffraction grating, 10 phase shift region, 15 passive waveguide layer, 18 oscillation longitudinal mode, 19 DBR reflection spectrum, 22 modulation region, 23 thin film heater.

Claims (4)

An active region having an active waveguide layer;
A DBR laser comprising a DBR mirror region provided adjacent to the active region and having a passive waveguide layer on an extension of the active waveguide layer;
The active part separated from the light output side end of the effective resonator length region by an integral fraction of the effective resonator length, which is the sum of the light penetration length into the DBR mirror region and the active region length. A DBR laser comprising a plurality of modulation regions for modulating the refractive index of light or the gain of a mode in a region in the region. The DBR mirror region is provided only at one end of the active region,
The DBR laser according to claim 1, wherein the other end of the active region is a cleavage end face.   The DBR laser according to claim 1, wherein the DBR mirror region is formed adjacent to both ends of the active region.   The DBR laser according to claim 1, further comprising a heater that heats the DBR mirror region.

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