Laser pump module with reduced tracking error

Details Laser pump module with reduced tracking error Laser pump module with reduced tracking error

2001-10-30

2004-09-14

Leung, Quyen (Department: 2828)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S033000

Reexamination Certificate

active

06792012

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to monitoring of light sources and more particularly to the suppression of tracking error in the monitoring of the output intensity of laser source, such as in the case of a laser pump module. However, this invention is equally applicable to any other applications where tracking of the intensity or power output of a laser source is required, albeit a semiconductor laser, a fiber laser or a solid-state laser.
BACKGROUND OF THE INVENTION
In the employment of pump laser modules, such as 980 nm and 1480 nm pump modules, for optical telecommunication applications, it is necessary to insure that the output intensity of the pump laser is maintained at a desired level. This is currently done by monitoring the output power that is provided out of the pump module pigtail fiber using a monitor device, such as a monitor photo diode (MPD), positioned at the back facet of the laser diode chip in the module package. The pump module typically comprises a laser diode chip with its front facet light output provided from the laser cavity aligned to be optically coupled into a single mode pigtail fiber which fiber terminates externally of the package for splicing to a fiber amplifier or fiber laser or other type of optical application. The optical coupling of the laser diode output has been accomplished by means of a lens that collimates and focuses the output light into the input end of the fiber. Some of the laser diode output light is reflected back from the lens back into the laser cavity, where it is amplified in the laser cavity and exits, in part, out of the back facet to the MPD. Another portion of the output light is scattered and lost within the module case or package. Light reflecting from the lens or other optical element may be detected directly without the light passing thorough the diode waveguide.
A more attractive approach for coupling this light is the use of a pigtail fiber that has a lens formed on its input end such as chisel or wedged shaped lens, as disclosed in U.S. Pat. Nos. 5,940,557 by Harker; 5,455,879 by Modavis et al.; 5,500,911 by Roff, and 5,074,682 by Uno et al.; all of which are incorporated herein by their reference. In particular, if the chisel shaped input end of the pigtail fiber is angled relative to the longitudinal axis of the fiber, further improvements in coupling efficiency can be realized as set forth in U.S. Pat. No. 5,940,557. The angled lens with an anti-reflecting (AR) coating placed on its surface prevents a significant portion of laser diode output light reflected off the input chisel lens from reentering the laser diode chip.
As is well known in the art of laser diodes, the back facet of the pump module laser diode has a high reflecting (HR) coating while the front facet has a low reflecting or anti-reflecting (AR) coating so that most of the laser diode optical power in the laser cavity will emanate from the front facet while being highly reflected at the back facet. However, a HR reflector is not a perfect reflector so that approximately 0.5% to 10% of the laser light will penetrate the HR coating and can be employed with the MPD to track the output power of the laser diode by sensing the back facet light from the laser diode. Another way of checking and monitoring the output power is split off a small portion of the output power, e.g. 0.5% or 1% and feed this small amount to an MPD. Typically, the monitor current is going to be about 0.5 to 1 milliamp of current per milliwatt of power from the laser diode chip back facet impinging on the MPD. It has been traditionally preferred to place the MPD at the back facet of the laser diode to take advantage of the small amount power emanating from the back facet of the diode.
One problem with the MPD detector in the package is that with changes in the ambient temperature within the module package for a given output power from the module, the MPD changes in value with such temperature changes. In use of the pump module, end users desire that, for a given MPD current output, a given optical output power can be derived from the module. However, there is always some variation to be expected with changes in the case temperature, but it is required to be within tolerable limits or range, which is now considered between about ±5-10% with a package temperature variation from about 0-75° C. In other words, a tracking error of MPD with ±8% is presently acceptable but values beyond this range are not generally acceptable to end users. Also, the maximum acceptable tracking error will likely be required to be reduced as end user's demands for higher accuracy continually increase, imposing further suppression of tracking error by pump module manufacturers. Tracking error herein is defined as the change in module output power with the change in case or package temperature for a fixed MPD current developed from the light output collected from the laser diode back facet by the MPD. We have experienced back facet MPD tracking errors in excess of this range and, therefore, something needs to be done to provide for more accurate tracking of the output power of the module to meet the need of end users.
There are several complicated factors in determining the cause of tracking error but two of the principal causes are described as follows. As the module case temperature changes with operation or with ambient temperature, the inside ambient of the pump module package, where the laser diode chip and MPD are positioned, is set to be at a predetermined operating temperature using a thermoelectric cooler (“TEC”), which may be any number of different operating temperatures but is typically 25° C. This is done so that the operating temperature remains the same so the optical characteristics of module operation do not significantly change with ambient temperature.
However, as the module package temperature changes during operation, the package, and particularly the platform supporting the laser diode and the coupling pigtail fiber input end, will flex or warp ever so slightly causing slight internal misalignment between the lensed fiber input tip or end and the laser front facet. This distance or cavity length between the fiber lens and the laser diode front facet is typically around 10 &mgr;m. Compared to the cavity length of the laser diode chip, this is quite small. The typical cavity length of a 98 nm chip is about 1.5 mm and the cavity length of a 1480 chip is about 2 mm.
The relative reflective feedback off the lensed fiber tip and the reflected light off of the external surface of the laser front facet form a Fabry-Perot (F-P) cavity. Thus, there are two such F-P cavities existing in the package—the laser Fabry-Perot (F-P) primary cavity and the facet-to-lens Fabry-Perot (F-P) secondary cavity wherein reflected light from these component surfaces in the secondary cavity achieves some degree of resonance. As the case temperature changes, the distance between the laser front facet and the fiber lens tip can change by a small amount.
Changes in the length of the secondary F-P cavity arising from changes in the case temperature causes the light in this secondary cavity to go into and out of phase with the phase of the light generated in the laser diode chip, adding to and subtracting from the light emitted from the laser diode. This change in phase does not have much effect on the pump module output power because the light reflected between the front facet of the laser diode and the face of the lensed fiber is relatively small compared to the total light output from the laser diode. However, these changes in phase interference can have a significant effect on the MPD because the feedback going into the laser diode from the secondary F-P cavity is amplified in the laser diode chip and the amplified output is detected by the MPD. Thus, the MPD detects a value that is not truly representative of the output intensity of the laser diode and the value detected by the MPD changes with the phase interference between the primary and secondary cavities even thoug

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