Method and apparatus for back facet monitoring of laser diode power output

The present invention presents an apparatus and method for efficiently and automatically monitoring the power output of laser diodes used to provide the scanning beams in a ROS system. Light radiated from the back facet of the laser diode is focused by a projection means, in a preferred embodiment, a ball lens onto a photosensor. The photosensor output is proportional to the power output of the diode associated with the photosensor. The use of a projection means provides immunity from stray light and from cross talk between closely positioned laser diodes in a multiple diode configuration and yield increase sensitivity of the photosensor.

BACKGROUND AND DISCLOSURE STATEMENT 
The present invention relates generally to the monitoring of output power 
of semiconductor laser diodes and particularly to the monitoring of the 
output power of laser diodes by detecting radiation emitted from the laser 
diode back facet. 
It is well known in the scanning art to use diode lasers to generate a 
coherent laser beam which is used to scan a recording medium surface. It 
is also known to use multiple laser diodes to create multiple beams, each 
individual beam independently modulated by video signals, and the multiple 
beams scanned onto the recording surface as modulated beams. For these 
multiple beam applications, it has been found advantageous to use arrays 
of closely spaced laser diodes. Closely spaced diodes allow for multiple 
beam processing and thus improve data throughout as compared with older 
systems that employ continuous wave, single beam gas or semi-conductor 
lasers. 
Typically, the laser diodes are individually addressable. Individual 
addressability generally requires that each diode have a separate current 
source that drives or modulates the diode. In operation, each driver sends 
a current through the diode sufficient to induce emission of laser light. 
The amount of current the driver produces is determined, in part, by the 
digital input data driving that particular lasing element. An example of a 
ROS system using a dual laser diode is disclosed in U.S. Pat. No. 
4,796,964, whose contents are hereby incorporated by reference. 
Because different laser diodes have different output power characteristics 
in response to a given driving current, it is desirable to monitor the 
amount of output power from each laser diode. If it is found that a 
particular diode is outputting too much or too little power at a given 
current level then the current needs to be adjusted to correct for the 
power differential. Laser diodes are typically constructed layer by layer 
from epitaxial deposition of appropriately doped semiconductor material. 
The front and back facets are then cleaved to produce reflective surfaces 
that define the front and back boundaries of the laser cavity. The front 
facet is designed to be more transmissive than the back facet which is 
generally made to be highly reflective. The front facet is thus the side 
from which the majority of laser light is emitted. 
As stated above, the back facet is frequently also designed to be a highly 
reflective surface. However, some light ultimately escapes through the 
back facet of the diode. The amount of light leakage through the back 
facet is generally known to be proportional to the amount of light emitted 
from the front facet. This relationship between radiation from the back 
facet and the radiation from the front facet affords the opportunity to 
monitor the amount of output power from the front facet by detecting light 
emitted from the back facet. 
To measure the amount of light from the back facet of a diode, a detector 
is typically disposed opposite the back facet of a single laser diode. In 
the case of a single laser diode configuration, one back facet detector 
gives complete information concerning the amount of radiation emanating 
from the front facet of that diode. In a multi-diode configuration, the 
confluence of concurrent, multiple beams does not give information 
concerning any particular diode. 
FIG. 1 shows a top perspective view of a prior art Raster Output Scan (ROS) 
system 12 which includes a single laser diode 15 whose output is monitored 
by back facet detection. The ROS scans a data modulated beam 13 onto a 
xerographic photoreceptor drum 14 in accordance with a predetermined 
raster scanning pattern. ROS 12 comprises a laser diode 15 which is driven 
in accordance with image signals entered into, and processed by, ESS 16. 
Laser 15 emits light beam 13. A polygon scanner 17 is optically aligned 
between laser 15 and the drum 14 and rotated so that facets 18 intercept 
the output beams and cause the beams to be swept across the drum surface 
in a fast scan direction. Pre-scan optics 20 and post-scan optics 22 
contain conventional optical elements which are used for beam forming and 
correction purposes. 
The laser diode 15 has front and back facets 15A, 15B, respectively. While 
the majority of the laser light escapes from the front facet as beam 13, 
some radiation in the form of beam 13' is emitted from the back facet of 
the diode. This radiation is detected by a photosensor 24 which generates 
an output signal which is sent to ESS 16. ESS 16 then processes this 
signal comparing it to a predetermined voltage level corresponding to the 
desired power output of the diode. If correction is needed, a signal is 
sent to the drive circuit for the laser to increase or reduce the laser 
power output. As can be seen from FIG. 1, the light from the front and 
back facets spreads out in a conic shape. Other prior art disclosures 
which utilize a back facet detection are found in U.S. Pat. Nos. 
4,342,050, 4,727,382 and 5,311,216. 
For a multiple diode configuration, a single back facet photosensor 
opposite the laser diodes cannot simultaneously provide discernible 
information concerning the output power of any one laser diode since the 
overlapping of concurrent multiple beams does not give information 
concerning any particular diode. While each laser diode can be monitored 
separately by alternately turning each diode on and off, it is more 
efficient to be able to separately and simultaneously measure the light 
intensity of each laser diode. 
Thus there is a need to construct an array architecture such that the 
amount of light emitted from individual back facets, of a multiple diode 
configuration is detected. Additionally, there is a need to regulate the 
output of the individual diodes in a continuous closed loop configuration 
to insure high print quality. 
It is thus a first object of the present invention to provide a back facet 
monitoring system such that the amount of output power from individual 
back facts of laser diodes can be individually monitored in a continuous 
fashion. 
There are additional prior art problems in back facet monitoring. In 
typical laser designs, up to 99.5% of the back facet is coated with a 
reflective material. Thus only a small amount of light (0.5%) is emitted 
from the back facet and is available to measure the power. Thus, 
photosensor 24 in FIG. 1, which is typically placed several millimeters 
behind laser 15, collects only a relatively small fraction of the already 
reduced light emitted from back facet 15B. 
It is therefore a second object of the present invention to increase the 
sensitivity of the back facet light detector. 
Another problem of prior art systems is stray light impinging on detector 
24 distorting the output signal. The stray light is the result of 
reflections from the optical components in the system (e.g., from the 
optical components in pre-scan optics 20) being reflected from the rear 
facet and onto the detector. The detected signal will be distorted due to 
the optically induced "noise". For multiple diodes, increased "cross talk" 
results. 
It is therefore a still further object of the invention to reduce the 
effects of stray light interference on back facet power monitoring 
detector signals. 
These and other objects are realized by introducing an imaging component 
between the back facet and a small area photosensor with a fast response 
time. In one embodiment, the back facet light emissions from a dual 
emittor diode array are imaged onto 2 small photosensors formed on an 
array. More particularly, the present invention relates to an apparatus 
for monitoring the power output of at least a laser diode having at least 
a front and back facet, said apparatus comprising: 
imaging means proximate said back facet and optically aligned with said 
back facet so that light emitted therefrom is imaged by said imaging means 
onto an imaging plane, and 
at least one photosensor optically aligned with said imaging means and 
disposed in said imaging plane wherein said imaged light is focused onto 
said photosensor generating an output signal therefrom, said output signal 
being proportional to said power output.

DESCRIPTION OF THE INVENTION 
The following description is presented to enable any person skilled in the 
art to make and use the invention, and is provided in the context of a 
particular application and its requirements. Various modifications to the 
preferred embodiments will be readily apparent to those skilled in the 
art, and the generic principles defined herein may be applied to other 
embodiments and applications without departing from the spirit and scope 
of the present invention as defined by the appended claims. Thus, the 
present invention is not intended to be limited to the embodiments shown, 
but is to be accorded the widest scope consistent with the principles and 
features disclosed herein. 
Referring to FIG. 2, a top view of one embodiment of the present invention 
having two laser diodes as shown. An array 30 is comprised of laser diodes 
32, 34. Array 30 can be used to provide beams, beams 40, 42 used for beam 
scanning purposes in a ROS system of the type disclosed, for example, in 
U.S. Pat. No. 4,796,964. Diode 32 has a front facet 32A and back facet 
32B; diode 34 has a front facet 34A and a back facet 34B. The light 
emitted from back facets 32B, 34B is focused onto photosensors 44, 46, 
respectively, by a focusing element which in a preferred embodiment is a 
ball lens 50. Photosensors 44, 46 are formed on a linear array 52. 
This arrangement collects an appreciable fraction of the light from the 
back facet of each laser and images the emitted light onto a photosensor. 
By collecting an appreciable fraction of the light emitted by each laser 
(30%-90% of the light emitted from back facets 32B, 34B) and imaging the 
light onto photosensors 44, 46, the maximum sensitivity of each sensor is 
realized. Since each back facet output is imaged onto a separate 
photosensor, the output of each diode can be detected independent of the 
other. 
Thus, each photosensor 44, 46 generates an output signal which is 
proportional to the power output of diodes 32, 34, respectively. The 
output signals are sent along electrical connection wires 60, 62 to ESS 70 
which contains the drive circuitry for each diode. The ESS compares output 
signals from the photosensor to signals representing optimum power levels 
for each laser diode and adjusts the driver signals to maintain this power 
output level. Since the individual diodes are focused, this permits the 
use of very small area photosensors thereby permitting them to have fast 
response times when operated under appropriate conditions. Finally, the 
small photosensor area and the focusing element at the rear of the laser 
means that any stray light that does not have an apparent origin the same 
as the laser array, will be spread over a large area and therefore 
contribute only a very small noise signal. Though the focusing element in 
FIG. 2 is a spherical ball lens, a diffractive or holographic lens or a 
reflective spherical mirror could be used among other choices to focus the 
back facet light output onto the photosensor. Since the spot shape and 
quality at the photosensor is not important, aberrations induced by the 
lens are not critical. The approach is compatible with hybrid laser arrays 
as well as lasers of arbitrary polarization. 
In a preferred embodiment, diodes 32, 34 are bonded to array 30 using 
conventional techniques. The ball lens 50 is soldered in place using a 
metal bonding pad that is placed on the lens in a standard commercially 
available configuration. The detector array 52 is made by silicon pin 
technology of a semi-custom pattern using standard processes. The 
optoelectronic package that the components shown in FIG. 2 would be seated 
in would be a semi-custom package. Additional features of the invention 
would be a window at the front of array 36 so that the laser diodes can be 
conventionally coupled to the pre-scan optics so that each laser can be 
driven independently and so that the photosensors in the array can be 
individually detected, and if speed is required, biased independent of the 
laser. The laser heatsink in base 74 is conventional and the bonding spot 
for the ball lens is moved from the conventional front facet position to 
the rear facet position. 
While a two diode laser array is described in the embodiment of FIGS. 2 and 
3, it is understood that the invention can be practiced with a single 
laser diode or with a multiple number of laser diodes; e.g. 4, 6. 
While the embodiment disclosed herein is preferred, it will be appreciated 
from this teaching that various alternative, modifications, variations or 
improvements therein may be made by those skilled in the art, which are 
intended to be encompassed by the following claims: