Semiconductor laser module

A semiconductor laser module comprises a laser diode that emits light in a frontward direction and a rearward direction, an optical fiber that transmits the light emitted in the frontward direction by the laser diode, and a monitor photodiode that receives the light emitted in the rearward direction by the laser diode. The monitor photodiode is characterized in that its sensitivity characteristic has a negative temperature coefficient.

FIELD OF THE INVENTION

The invention relates to a semiconductor laser module comprising an optical fiber and a laser diode which are optically coupled to each other, and a monitor photodiode which receives light emitted from the rear side of the laser diode.

BACKGROUND OF THE INVENTION

FIG. 8is a block diagram showing the construction of a conventional semiconductor laser module. The conventional semiconductor laser module includes a laser diode (LD)1, an optical fiber2for transmitting the light emitted from the front side of the laser diode1, and a lens3disposed between the LD1and the optical fiber2for converging the light emitted from the front side of the laser diode1. Also, a monitor photodiode (PD)4is disposed on the rear side of the LD1to monitor the light emitted from the rear side of the LD1, and an APC (automatic power control) circuit5is used for controlling the driving current of the LD so as to make a monitor PD current constant to keep output of the optical fiber constant.

Conventionally, in semiconductor laser modules for dealing with a short wavelength band around 980 nm, Si—PD having a high sensitivity concerned above wavelength band have been used as a monitor PD.

FIG. 9shows a relationship between LD operating current and fiber end optical output of a semiconductor laser module of 980 nm band using Si—PD. InFIG. 9, optical output from the front side due to temperature of the LD is changed to 72.9 mW at 5° C. and 66.1 mW at 45° C. when an operating current is 150 mA, that is, a reduction of 0.43 dB occurs (=10×LOG (66.1/72.9)).

FIG. 10shows measurement results of sensitivity characteristic of Si—PD relative to temperature. Data inFIG. 10indicates an increase of 0.25 to 0.3 dB between 5° C. and 45° C. That is, the sensitivity characteristic of Si—PD presents a positive temperature coefficient, at which the sensitivity is enhanced as temperature rises.

FIG. 11shows a relationship between monitor PD current and fiber end optical output of a semiconductor laser module of 980 nm band using an Si—PD. When a monitor operating current of Si—PD is 0.116 mA, the optical output is 72.9 mW at 5° C. and 62.8 mW at 45° C., that is, a reduction of 0.65 dB (=10×LOG (62.8/72.9)) occurs. This indicates a tracking error of a semiconductor laser module, and means a decline above temperature change of the LD as apparent from comparison with FIG.9.

Reasons for the tracking error in a semiconductor laser module include a change in reflection coefficients of an LD on front and rear sides due to temperature, deviation of optical axis of the optical fiber. Usually, the reflection coefficient of the LD is as low as several percents on the front side but is as high as 90% or higher on the rear side, whereby change is large due to temperature in optical output from the front side of an LD but is little on the rear side of an LD. Therefore, in a case of performing an APC control with a constant monitor PD current, a large amount of tracking error occurs due to temperature change. That is, a decline in optical output from the front side inFIG. 9substantially corresponds to that. However, in the case of using Si—PD, the PD sensitivity is enhanced by temperature rise as shown inFIG. 10, so that even upon reception of the same optical output the monitor PD current becomes large with temperature rise. This current is fed back to an APC circuit in which the monitor PD current is controlled to be made constant, whereby the APC circuit operates in a direction of decreasing an LD operating current (direction of decreasing an optical output) to thereby make a tracking error further larger.

As described above, conventional semiconductor laser modules of 980 nm band use an Si—PD for APC circuits, and so a tracking error due to temperature change becomes larger than that due to an LD itself.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a semiconductor laser module capable of reducing a tracking error due to temperature change more as compared to that due to an LD itself.

The semiconductor laser module according to the invention comprises a laser diode which emits light in a frontward direction and a rearward direction, an optical fiber which transmits the light emitted in the frontward direction by the laser diode, and a monitor photodiode which receives the light emitted in the rearward direction by the laser diode. The sensitivity characteristic of the monitor photodiode has a negative temperature coefficient.

DETAILED DESCRIPTION

FIG. 1shows a block diagram of a semiconductor laser module according to an embodiment of the invention. The semiconductor laser module according to the embodiment of the invention is provided with an LD1and an optical fiber2. A monitor PD14using InGaAs—PD is disposed on the rear side of the LD1to monitor light emitted from the rear side of the LD1. An APC circuit5is used for controlling driving current of the LD so as to make a monitor PD current constant to keep output of the optical fiber constant.

FIG. 2shows measurement results of sensitivity characteristic of InGaAs—PD relative to temperature. InGaAs—PD has a negative temperature coefficient to be decreased in sensitivity as temperature rises, and data inFIG. 2indicates a decline of 0.2 to 0.25 dB between 5° C. and 45° C.

FIG. 3shows a relationship between LD operating current and fiber end optical output of the semiconductor laser module of 980 nm band using InGaAs—PD.FIG. 4shows a relationship between monitor PD current and fiber end optical output of the semiconductor laser module of 980 nm band using InGaAs—PD.

InFIG. 3, optical output from the front side due to temperature of the LD is changed to 78.3 mW at 5° C. and 71.2 mW at 45° C. when an operating current is 150 mA, that is, a reduction of 0.41 dB (=10×LOG (71.2/78.3)) occurs. Since the InGaAs—PD receiving the light emitted rearward of the LD has a temperature coefficient whose sensitivity characteristic is negative, the monitor current is decreased corresponding to decline in sensitivity for the light emitted from the rear side of the LD which is little varied due to temperature rise. The APC circuit will control an operating current of the LD in a direction of increasing the current with respect to the decreased monitor current.

InFIG. 4, when a monitor current of the InGaAs—PD is 0.395 mA, the optical output is 78.3 mW at 5° C. and 74.8 mW at 45° C. to decrease as low as 0.2 dB (=10×LOG (74.8/78.3)). This indicates a tracking error of the module, and the tracking error is reduced as compared with the decline due to temperature change of the LD.

The semiconductor laser module according to the invention may be one using a core-expanded fiber12shown in FIG.5. The use of the core-expanded fiber can mitigate tolerance due to deviation of optical axis and is advantageous in occurrence of deviation of optical axis due to temperature change, further enabling enhancement in manufacture yield.

A wedge-shaped fiber22with a tip end formed in wedge-shaped form shown inFIG. 6may be used. This can enhance coupling efficiency in the case of LD emitting elliptical-shaped light, and omit the lens3, by which low cost can be expected due to reduction in the number of parts.

An optical fiber32to which a fiber grating33as shown inFIG. 7is applied, may be used. Thereby, it is possible to realize a semiconductor laser module in which oscillation spectrum of LD can be controlled to stably provide a desired wavelength.

As described above, with the semiconductor laser module according to the invention, the InGaAs—PD having a temperature coefficient whose sensitivity characteristic is negative, is used for a monitor PD receiving the light emitted from the rear side of the LD, and therefore a tracking error can be reduced more relative to decline of the LD itself due to temperature change.