JPH1075004A - Semiconductor light-emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase an allowance for a change in current, a change in temperature, etc., and enable a stable oscillation at a single mode in a semiconductor light-emitting device which selects an emission wavelength by a band pass filter having a narrow transmission wavelength band characteristic. SOLUTION: In a semiconductor light-emitting device, wherein a light beam 11 emitted from an end face 10b of a semiconductor optical amplifier 10 is passed through an interference filter 14 having a narrow transmission wavelength band characteristic for selecting its wavelength, and the light beam 11 is reflected by a mirror 13 to be sent back to the semiconductor optical amplifier 10, such an interference filter 14 is used whose transmission band width BW (the full width at half maximum intensity when the beam passes twice: the unit is nm) is smaller than (dλ0 -0.025)/0.214, where dι0 (the unit is nm) is an interval in the Fabry-Perot internal mode of the semiconductor optical amplifier 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device provided with a semiconductor optical amplifier as a light emitting source, for selecting light emitted from the semiconductor optical amplifier and returning the light to the semiconductor optical amplifier. To control the emission wavelength to a desired value.

[0002]

2. Description of the Related Art Conventionally, various attempts have been made to obtain a single-wavelength high-power light beam using a semiconductor. ELEC
TRONICS LETTERS Vo
l.29, No. 14, (1993) pp. 1254-1255 shows one such semiconductor light emitting device.

As shown in FIG. 12, this semiconductor light emitting device has a semiconductor optical amplifier 1 as a light emitting source. After the light emitted from the rear end face 1a of the semiconductor optical amplifier 1 is collimated by a lens 2, it is reflected. The light is reflected by the diffraction grating 3 and returned to the semiconductor optical amplifier 1. In this configuration, the light 4 whose wavelength is selected by the diffraction grating 3 is returned to the semiconductor optical amplifier 1 so that the wavelength of the light 4F emitted from the front end face 1b is locked to a single wavelength. Further, in this semiconductor light emitting device, the emission wavelength can be changed within a certain range by adjusting the installation angle so that the light incident angle on the diffraction grating 3 changes.

A conventional semiconductor light-emitting device that selects an emission wavelength by using this external optical system utilizes the fact that the diffraction angle of the diffraction grating changes according to the wavelength. A spatial aperture that cuts off a light beam that travels off a predetermined optical path after reflection is required. Conventionally, as shown in FIG. 12, a taper stripe amplifier in which the stripe width Wi on the external optical system side is reduced to, for example, 4 μm as the semiconductor optical amplifier 1 is used, and this narrow stripe is used as a substantial spatial aperture. However, if there is a restriction that a semiconductor optical amplifier having such a narrow stripe width must be used, it is difficult to meet the demand for higher output and the like.

Further, in the above-mentioned conventional semiconductor light emitting device, it is necessary to set the angle of the diffraction grating to a predetermined angle with high accuracy in order to obtain a desired emission wavelength, and the adjustment operation is very troublesome and difficult. Becomes Further, since the diffraction grating is generally quite expensive, it is difficult to manufacture the semiconductor light emitting device at low cost.

[0006] In order to solve such a problem, the present applicant has previously made it possible to select an emission wavelength by an external optical system, to have a high degree of freedom in selecting a semiconductor optical amplifier, and to meet the demand for cost reduction. A semiconductor light emitting device which can be easily adjusted is proposed (JP-A-8-32161).

In this semiconductor light emitting device, a light emission wavelength is selected by a bandpass filter instead of the above-described diffraction grating. That is, a semiconductor optical amplifier and light emitted from one end surface of the semiconductor optical amplifier are reflected. And a mirror for returning to the one end face, and a bandpass filter having a narrow transmission wavelength band characteristic inserted in the optical path of the light.

In this semiconductor light emitting device, the light emitted from one end face of the semiconductor optical amplifier is selected in a narrow wavelength band by a band-pass filter, and the light is returned to the semiconductor optical amplifier. As in the case, the emission wavelength of the semiconductor optical amplifier is controlled to a single wavelength.

In this semiconductor light emitting device, since the wavelength selection is performed only by the band pass filter, no spatial aperture is required. Therefore, it is not necessary to limit the use of a semiconductor optical amplifier having a narrow stripe width, and various semiconductor optical amplifiers including a high-output type can be selected.

In general, a band-pass filter can be manufactured at a lower cost than a diffraction grating, so that this semiconductor light emitting device can be formed at a lower cost than a conventional semiconductor light emitting device using a diffraction grating.

[0011] Unlike a diffraction grating, a bandpass filter does not obtain wavelength selectivity by angular dispersion. Therefore, it is not necessary to strictly adjust the angle of the bandpass filter for selecting an emission wavelength. Therefore, the semiconductor light emitting device can be easily adjusted.

[0012]

As described above, Japanese Patent Application Laid-Open No.
The 32161 semiconductor light emitting device can solve the above-mentioned various problems, but on the other hand, the oscillation mode changes due to a current change, a temperature change, or the like. Current control and temperature control were required.

The manner in which the oscillation mode changes in a conventional semiconductor light emitting device that selects an emission wavelength using a bandpass filter having a narrow transmission wavelength band characteristic will be described in detail below. FIG. 2 shows an example of a conventional semiconductor light emitting device using a band-pass filter. This semiconductor light emitting device has a semiconductor optical amplifier having a tapered stripe 10a.
10, a collimator lens 12 for collimating the light beam 11 emitted from the rear end face 10b of the semiconductor optical amplifier 10, and reflecting the collimated light beam 11 back to the original optical path, for example, Al Mirror 13 and this mirror
A narrow-band interference filter 14 inserted into the optical path of the light beam 11 between the collimator lens 13 and the collimator lens 12 constitutes a confocal optical system with the collimator lens 12, and the parallel collimated light beam 11 is placed on the mirror 13. And a converging lens 15 that converges at.

As shown in FIG. 3, for example, the width Wr of the tapered stripe 10a on the rear end face 10b side is 4 μm, the width Wf on the front end face 10c side is 160 μm, and the width of the rear end face 10b and the front end face 10c is as shown in FIG. Both have a reflectance of 0.1% or less and a length (resonator length) L of 1500 μm.

FIG. 5 shows the dependence of the oscillation wavelength on the driving current when a narrow-band transmission filter having the spectral transmission characteristic of F2 shown in FIG. 4 is used as the narrow-band interference filter 14 in the semiconductor light emitting device described above. As shown here, when the driving current increases, the oscillation wavelength shifts to the longer wavelength side due to a rise in the temperature of the semiconductor junction, but oscillates with a substantially single spectrum due to the effect of the narrow band interference filter 14.

At this time, when the light makes one round trip in the external resonator, that is, in a range narrower than the full width at half maximum of the transmission wavelength band when the light passes through the filter 14 twice, the wavelength changes in a zigzag manner. This is the end face 10b of the semiconductor optical amplifier 10.
And 10c are formed to a reflectance of 0.1% or less, but there is a residual Fabry-Perot mode inside the semiconductor optical amplifier, and the mode hops to the next internal mode on the short wavelength side while being controlled by the filter 14. Is attributed to

The above conventional semiconductor light emitting device has a wide full width at half maximum of, for example, 0.01 nm or more when oscillating at a single wavelength, and has poor spectral purity. Also, F in FIG.
Even when the oscillation spectrum is further stabilized by using the first filter, the range of the current oscillating in the single mode and the device temperature are narrow, and the far-field pattern largely changes as shown in FIG. I do.

Particularly, a change in the far-field image is a disadvantage that seriously hinders a laser beam printer, a laser beam scanner, and the like. Since the semiconductor optical amplifiers used in these devices are formed for high output and the beam emission width is generally as large as 50 μm or more, the far field of the beam close to the diffraction limit has a divergence angle of 1 degree or less as shown in FIG. Becomes a strong beam. In practice, the light is often once condensed on a slit or the like and used after cutting the peripheral side lobes. However, when the beam is deformed at this time, the amount of light transmitted through a spatial filter such as the slit fluctuates. As described above, if the intensity variation occurs simultaneously with the deformation of the beam shape, it becomes a serious problem in application of the semiconductor light emitting device.

The present invention has been made in view of the above circumstances, and uses a band-pass filter having a narrow transmission wavelength band characteristic to select an emission wavelength. It is an object to provide a semiconductor light emitting device that stably oscillates in a mode.

Another object of the present invention is to provide a semiconductor light emitting device which satisfies the above-mentioned requirements and further has a small change in the far-field pattern of the oscillation beam.

[0021]

As described above, a first semiconductor light emitting device according to the present invention comprises a semiconductor optical amplifier and a mirror for reflecting light emitted from one end of the semiconductor optical amplifier and returning the reflected light to the one end. And a band-pass filter having a narrow transmission wavelength band characteristic inserted in the optical path of the light, wherein the band-pass filter selects an emission wavelength. When the interval is dλ _o (unit is nm), the transmission band width BW (2
The full width at half maximum at the time of single transmission: the unit is nm, but (dλ _o −0.025)
) /0.214.

A second semiconductor light emitting device according to the present invention is characterized in that, in the semiconductor light emitting device having the above-described configuration, the reflectance of the end face of the semiconductor optical amplifier opposite to the one end face is 1% or more. Things.

[0023]

Embodiments of the present invention will be described below in detail. FIG. 1 shows a semiconductor light emitting device according to a first embodiment of the present invention. As shown, the semiconductor light emitting device includes a semiconductor optical amplifier 10 having a tapered stripe 10a, and a rear end face 10b of the semiconductor optical amplifier 10.
A collimator lens 12 for collimating the light beam 11 emitted from the light source, a mirror 13 made of Al for reflecting the collimated light beam 11 back to the original optical path, and a mirror 13 and a collimator lens 12 made of Al. Between the light beam 11
And a condensing lens 15 that forms a confocal optical system with the collimator lens 12 and that converges the parallel light beam 11 on the mirror 13.

The planar shape of the semiconductor optical amplifier 10 is the same as that shown in FIG. 3, and the width Wr of the tapered stripe 10a on the rear end face 10b side is 4 μm, and the width Wf on the front end face 10c side is Wf.
= 160 μm and length (resonator length) L = 1.5 mm. The collimator lens 12 has a numerical aperture NA = 0.6 and a focal length f = 6 mm. On the other hand, the focal length f of the condenser lens 15 is 25.6 mm. As the interference filter 14,
F1 and F2 in FIG. 4 are used for comparison. The interference filter 14 is not placed perpendicularly to the optical path but tilted several degrees in order to prevent light directly reflected from the surface from returning to the semiconductor optical amplifier 10. This angle can be selected appropriately.

The semiconductor optical amplifier 10 is, for example, an n-GaAs substrate (Si = 2 × 10 ¹⁸ cm ⁻³ doped).
An n-GaAs buffer layer (Si = 1 × 10 ¹⁸ cm ⁻³ doped, layer thickness 0.5 μm), an n-Al _0.5 Ga _0.5 As clad layer (Si = 1 × 10 ¹⁸ cm ⁻³ doped, layer thickness 2.5 μm)
m), n-Al _0.25 Ga _0.75 As optical guide layer (undoped, layer thickness 0.05 μm), n-Al _0.05 Ga _0.95 As quantum well layer (undoped, layer thickness 8 nm), n-Al _0.25 Ga
_0.75 As light guide layer (undoped, layer thickness 0.05 μm), p
-Al _0.5 Ga _0.5 As clad layer (Zn = 1 × 10 ¹⁸ c
A layer structure is used in which a m- ³ dope, layer thickness 2 μm) and a p-GaAs cap layer (Zn = 5 × 10 ¹⁸ cm ⁻³ dope, layer thickness 0.3 μm) are formed by a reduced pressure MOCVD method.

As the tapered stripe 10a, for example, SiO 2 is formed on the cap layer by a plasma CVD method.
₂ films are formed, the SiO ₂ film is removed by photolithography and etching in a tapered region serving as a stripe, and ohmic electrodes are formed on the p− side by AuZn / Au and on the n− side by AuGe / Au. Can be used.

In this embodiment, the semiconductor optical amplifier 10
The end faces 10b and 10c are coated with a low reflectance coating,
Thereby, the reflectance Rr of the end face 10b and the reflectance R of the end face 10c
f are each set to 0.1% or less.

Here, the dependence of the oscillation wavelength on the driving current as shown in FIG. 5 was measured, and the wavelength width (the amplitude of the zigzag change) in which the same longitudinal mode was maintained was measured. FIG. 7 shows the change wavelength width Δλ and the transmission bandwidth BW (1
(Reciprocation = full width at half maximum when transmitted twice). From this relationship, it has been found that the change wavelength width Δλ is smaller than the transmission bandwidth BW of the interference filter 14. That is, Δλ is represented by a linear function of BW, and the upper limit of the change wavelength width Δλ shown by the straight line A in FIG. 7 is Δλ = 0.025 + 0.214 × BW (nm) (a).

On the other hand, in order for the semiconductor light emitting device to operate effectively in a single mode, if the interval between the Fabry-Perot internal modes of the semiconductor optical amplifier 10 is dλ _o (unit is nm), Δλ <dλ _o ... The relationship of (b) is required.

From the above relationships (1) and (2), if BW <(dλ _o −0.025) /0.214 (c), the semiconductor light emitting device operates effectively in a single mode.

More specifically, the distance dλ _o between the Fabry-Perot internal modes is, as is well known,

[0032]

(Equation 1)

(Eg, HCCasey, Jr. and
MBPanish “Heterostructure Lasers, PART A”, Academ
ic Press, New York, p. 167 (1978)). Where λ _o
Is the oscillation wavelength, n is the equivalent refractive index of the propagation mode in the optical amplifier,
dn / dλ _o is the wavelength dispersion of the equivalent refractive index. Therefore, the necessary condition of the above (c) is:

[0034]

(Equation 2)

## EQU1 ## Therefore, if the resonator length L of the semiconductor optical amplifier 10 and the transmission bandwidth BW of the interference filter 14 are set so as to satisfy the expression (2), the semiconductor light emitting device can operate effectively in a single mode. Become. As an example, BW = 0.072 nm, n = 3.4, λ _o = 800 nm,
When dn / dλ _o = 6.0 nm ⁻¹ , the aforementioned L = 1.5 m
The value of m satisfies the equation (2).

Next, as a further preferred embodiment of the present invention, the reflectivity Rf of the front end face 10c of the semiconductor optical amplifier 10 is optimized, and Rf = 3.7%, which is higher than that of a normal semiconductor optical amplifier. On the other hand, as the interference filter 14, F1 in FIG.
Those having the following characteristics were used.

Here, for comparison, the reflectance of the front end face Rf ≦ 0.1% (0.02 to 0.09) is compared with that of the preferred embodiment.
%), The drive current dependence of the spectrum was compared. The results are shown in comparison with FIGS. From here, by increasing the front end face reflectance Rf, a single spectrum region is expanded,
It has been found that the spectral line width during single mode oscillation becomes narrower.

FIG. 9 shows the drive current dependency of the far-field image in the semiconductor light emitting device of the preferred embodiment. As shown here, an extremely stable light beam was obtained without any change depending on the drive current of the far-field image.

Next, the front end face reflectance Rf of the semiconductor light emitting device
The relationship between and the spectral line width Δν will be described in detail. FIG. 10 shows the spectral line width Δν at the time of single mode oscillation in the preferred embodiment as a function of the front end face reflectance Rf. The spectral line width Δν of a general semiconductor laser is represented as follows (Kenichi Iga, edited by the Japan Society of Applied Physics, “Semiconductor Laser” Ohmsha, p. 30)
7 (1994)).

Δν = R _spon (1 + α ² ) / 4πS (d) where R _spon is the spontaneous emission rate, S is the average number of photons in the resonator, and α is the spectral line width increase coefficient. R _spon and S are respectively

[0041]

(Equation 3)

Is represented by Where vg is the group velocity, n _sp is the spontaneous emission coefficient, α _m is the mirror loss, α _L is the internal loss,
P _o is the light output from the one end face, hv represents the photon energy of the oscillation mode. Therefore, equation (d) uses the mirror loss α _m

[0043]

(Equation 4)

The mirror loss α _m is 1 og _e (1 / Rf · R
r) / 2L, hν is a function of the end face reflectivity. Internal loss α of the semiconductor optical amplifier of the present embodiment
_L is α _L = 5 cm ^{−1 based} on the measurement of the resonator length dependence of an element manufactured as a semiconductor laser. (Equation 4)

[0045]

(Equation 5)

Since the following relationship is obtained, α _m (α _m +5) is plotted in FIG. 10 as a function of Rr, using Rr as a parameter. The scale on the vertical axis was selected so as to match the value of Δν measured here (marked by ●). From the comparison between FIG. 10 and the calculation, it can be seen that the equivalent reflectance of the rear end face as an external resonator is as low as 1% or less. This is because the efficiency of the lens system is about 25 to 30% in a round trip, and the transmittance of the interference filter 14 is not high as shown in FIG. From this result, the average round trip during oscillation is estimated to be about two (because (0.3 × 0.3) ^1.91 = 0.01).

The spectral line width Δν changes gradually with respect to the front end face reflectance Rf, and changes approximately logarithmically. From the tendency shown in FIG. 10 and the experimental results, it is considered that the oscillation mode is stabilized when the spectral line width Δν is about 0.007 nm or less. Therefore, in order to stabilize the oscillation mode, the front end face reflectance Rf is 1%. It is preferable to make the above.

In the above example, a gain-guided optical amplifier is used as the semiconductor optical amplifier, but a refractive-index-guided optical amplifier having a refractive index step at the boundary between both side surfaces of the stripe is used. You can also. In addition to a simple tapered optical amplifier, it is also possible to use another type of semiconductor optical amplifier such as an array type in which a plurality of stripes are arranged.

On the other hand, the optical system of the external resonator is not limited to the confocal optical system shown in FIG. For example, as shown in FIG. 11, it is possible to change the optical system such as disposing an external mirror at the collimated beam position.

[0050]

As described above in detail, the first semiconductor light emitting device according to the present invention, assuming that the interval between the internal modes of the Fabry-Perot mode of the semiconductor optical amplifier is dλ _o (nm), serves as a band-pass filter and a transmission band. When the width BW (nm) is (d
By using a material smaller than the value of λ _o −0.025) /0.214, it is possible to stably oscillate in a single mode.

In the second semiconductor light emitting device according to the present invention, in addition to the above effects, by setting the front end face reflectance of the semiconductor optical amplifier to 1% or more, a single spectrum region is expanded, and The spectrum line width at the time of single mode oscillation is sufficiently narrow. For this reason, the tolerance to external temperature fluctuations and current changes increases, and it becomes possible to use simpler drive circuits and temperature control circuits.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a semiconductor light emitting device according to an embodiment of the present invention.

FIG. 2 is a schematic plan view showing an example of a conventional semiconductor light emitting device.

FIG. 3 is an enlarged plan view of a semiconductor optical amplifier used in the semiconductor light emitting device of FIG. 2;

FIG. 4 is a graph showing a spectral transmittance characteristic of an interference type narrow band transmission filter used in the semiconductor light emitting device of FIG. 1;

FIG. 5 is an explanatory diagram showing a drive current dependency of an oscillation wavelength in a semiconductor light emitting device.

FIG. 6 is an explanatory diagram showing a drive current dependency of a far-field image in a conventional semiconductor light emitting device.

FIG. 7 shows a variation wavelength width Δ in the semiconductor light emitting device of the present invention.
Graph showing the relationship between λ and the transmission bandwidth BW of the interference filter

FIG. 8 is an explanatory diagram showing the drive current dependence of the oscillation spectrum of the semiconductor light emitting device.

FIG. 9 is an explanatory diagram showing the drive current dependence of the far-field image in the semiconductor light emitting device of the present invention

FIG. 10 is an explanatory diagram showing the relationship between the front end face reflectance and the oscillation spectrum line width of the semiconductor optical amplifier.

FIG. 11 is a schematic plan view showing another semiconductor light emitting device according to the present invention.

FIG. 12 is a schematic plan view showing another conventional semiconductor light emitting device.

[Explanation of symbols]

 10 Semiconductor optical amplifier 11 Light beam 12 Collimator lens 13 Mirror 14 Interference filter

Claims (2)

[Claims] 1. A semiconductor optical amplifier, a mirror that reflects light emitted from one end face of the semiconductor optical amplifier and returns the light to the one end face, and a bandpass filter having a narrow transmission wavelength band characteristic inserted into an optical path of the light. In the semiconductor light emitting device, when the interval between the Fabry-Perot internal modes of the semiconductor optical amplifier is dλ _o (unit is nm), the band-pass filter has a transmission bandwidth BW (full width at half maximum when transmitted twice). : A semiconductor light emitting device characterized in that a device having a unit (nm) smaller than (dλ _o −0.025) /0.214 is used. 2. The semiconductor light emitting device according to claim 1, wherein a reflectance of an end face of said semiconductor optical amplifier opposite to said one end face is 1% or more.