Apparatus and method for controlling the light intensity of a laser diode

A closed loop circuit for controlling the intensity of light emitted by a laser diode. A portion of the light emitted by the laser diode is used as an optical feedback signal and applied to a photodetector. The difference between a current produced by the photodetector and a reference current is an error current which is passed via a low impedance path to an integrating amplifier. The integrated error current is used to control the current flowing through the laser diode, thereby controlling the intensity of light emitted.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of laser diodes and, more 
particularly, to circuitry for delivering a light beam of controllable 
intensity from a laser diode. 
2. Discussion of the Prior Art 
A laser diode is a device which emits coherent light whose intensity is 
approximately proportional to the current flowing through the diode over a 
20:1 dynamic range. A laser diode's light intensity may be effectively 
modulated by time-varying the current through the diode. Such modulation 
is useful in various applications including, for example, imaging systems 
where a laser diode is used as a light source for the pixel-by-pixel 
recording of continuous tone images. 
A laser diode's operating characteristics are temperature-dependent and may 
vary widely in response to changes in the ambient temperature or 
self-heating. Such temperature changes may cause significant changes in 
the intensity of emitted light even though the current through the diode 
is constant. 
A servo loop circuit may be used to control and stabilize the light emitted 
by a laser diode. A portion of the light emitted by the diode is used as a 
feedback signal and applied to a photodetector. The magnitude of a signal 
produced by the photodetector is related (i.e., proportional) to the 
intensity of the light emitted by the laser diode. By comparing the signal 
produced by the photodetector with an input signal (reference), a 
difference or error signal is obtained which may be used to adjust the 
current flowing through the laser diode and, in turn, the intensity of 
light emitted. 
One problem which arises with such feedback circuits is that the loop gain 
decreases when the current flowing through the laser diode is relatively 
small (i.e., low light intensity), because the laser diode is operating in 
a non-linear region. In this region, relatively large changes in the 
magnitude of the current through the laser diode produce only small 
changes in the intensity of light emitted by the laser diode (and thus 
small changes in the feedback signal). Consequently, it is desirable to 
integrate the error signal to force the error to zero, which has the 
effect of trading off loop gain for bandwidth. 
However, conventional integrating feedback circuits may not provide 
sufficient bandwidth for applications in which the input signal varies 
rapidly over a wide dynamic range. One such application, referenced above, 
is continuous tone imaging, which typically requires extremely rapid 
modulation of the light emitted by the laser diode over a large dynamic 
range. 
An additional problem arises when a photodiode (typically a PIN diode is 
chosen) is used as the photodetector in the feedback loop. Because of the 
diode junction capacitance the voltage drop across the photodiode is a 
function of frequency, significant error may be introduced into the loop 
where that voltage drop is used as a feedback signal and there is 
significant variation in frequency. 
In one prior arrangement, the current from the diode passes through a 
resistor and the resulting voltage across the resistor is applied to a 
unity gain amplifier. The output of this amplifier is the feedback signal 
for the servo loop. 
The output of the unity gain amplifier is also fed back to the photodiode, 
ideally to maintain a constant zero (AC) voltage across the photodiode (in 
order to cancel the effects of the photodiode's junction capacitance). 
However, the amplifier has a finite gain bandwidth product, which means 
that at higher frequencies a non-zero voltage will appear across the 
photodiode and error will be introduced into the system. In addition, at 
high frequencies the unity gain amplifier requires a high input current 
and is generally noisy. 
SUMMARY OF THE INVENTION 
In brief summary, the present invention provides a closed loop circuit or 
servo for controlling the intensity of light emitted by a laser diode 
which is modulated by an analog input signal. The invention provides 
substantially improved bandwidth when operating the laser diode at low 
light levels. In an exemplary embodiment, the servo loop provides a -3 db 
bandwidth of approximately 50 MHz over a 20:1 dynamic range. 
The servo continuously adjusts the current flowing through the laser diode, 
thus adjusting the intensity of emitted light to correspond with the input 
signal. A portion of the light emitted by the laser diode is used as a 
feedback signal and applied to a photodetector, which produces a current 
that is proportional to the intensity of the impinging light. The 
difference between the input signal current and the current produced by 
the photodetector is an error signal (current) that is integrated by a 
transimpedance amplifier, which produces a voltage control signal at its 
output. The control signal is buffered and applied to a wide-band 
transconductance amplifier which controls the current flowing through the 
laser diode. 
The transimpedance amplifier preferably includes an operational amplifier, 
a single-stage discrete amplifier driven by the operational amplifier and 
a parallel combination resistor and capacitor which provide a load 
impedance for the discrete amplifier. These elements cooperate to perform 
the integration function over a wide frequency range of error signals. At 
lower frequencies, the operational amplifier, in combination with a 
feedback capacitor coupled between its output and inverting input, 
integrates the error current. The integrated error current is converted to 
a voltage by the load impedance of the discrete amplifier. 
At higher frequencies, the feedback capacitor coupled to the operational 
amplifier appears as a short circuit that effectively bypasses the 
operational amplifier and applies the error current directly to the 
emitter of the discrete amplifier. The discrete amplifier thus operates in 
a common base configuration, which is characterized by a low input 
impedance, a high output impedance and a current gain close to unity. The 
high frequency components of the error signal are thus effectively 
integrated by the load capacitor. 
By using the current generated by the photodetector, as opposed to the 
voltage drop across the photodetector, as a feedback signal, error due to 
frequency-dependent changes in the voltage drop is substantially 
eliminated. 
In addition, since the error signal (current) is integrated over a broad 
frequency band, the laser diode characteristics are effectively linearized 
over that band, thus providing more precise control of the laser diode's 
light intensity even at high input signal frequencies.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
FIG. 1 shows a closed loop circuit or servo 2 for controlling the intensity 
of light emitted by a laser diode which is modulated by an analog input 
signal. A multiplying digital to analog converter (DAC) 4 is connected to 
receive digital data and a multiplier value as input signals. The 
resulting analog output of the DAC 4 is connected via a resistor 6 to the 
anode of a photodetector or photodiode 10 and to the inverting input of an 
operational amplifier 8. The cathode of the photodiode 10 is connected to 
a source of potential +V, which serves to reverse bias the photodiode. The 
non-inverting input of the amplifier 8 is grounded. A capacitor 15 is 
connected between ground and the output of the amplifier 8, which is also 
connected to the base of a transistor 14. A feedback capacitor 12 is 
connected between the emitter of transistor 14 and the inverting input of 
amplifier 8. A resistor 16 is connected between the emitter of transistor 
14 and a source of potential -V. 
A load impedance comprising the parallel combination of a resistor 18 and a 
capacitor 20 is connected between ground and a node 21 at the collector of 
transistor 14 A constant current source 22, which supplies a current 
I.sub.b, is connected between the potential source +V and the node 21. An 
input of a buffer amplifier 24 is also connected to the node 21. 
For convenience and clarity, the resistance or capacitance of a component 
will be expressed as "R" or "C," respectively, having a subscript which 
corresponds with the reference numeral of the component. As explained in 
detail below, the product R.sub.18 C.sub.20 is preferably equal to the 
product R.sub.16 C.sub.12. 
The components within the dashed line denoted by the reference numeral 7 
are referred to collectively hereafter as a transimpedance integrating 
amplifier 7. 
The output of buffer amplifier 24 is connected to the base of a transistor 
30. A resistor 28 is connected between the emitter of the transistor 30 
and the potential source +V. The resistor 28 and transistor 30 operate 
collectively as a transconductance amplifier 26. 
The anode of a laser diode 32 is connected to the collector of the 
transistor 30 and the cathode of diode 32 is grounded. Light emitted by 
the laser diode 32 passes through a beam splitter 34. A portion of the 
light passing through the beam splitter 34 passes along an optical path 36 
and impinges on the photodiode 10. The bulk of the light entering the beam 
splitter 34 passes through the beam splitter as an output beam. 
Transistors 14 and 30 are preferably of the type which are designed to 
operate over a frequency range extending to 1-2 GHz, such transistors 
being commercially available from a number of sources. Buffer amplifier 24 
is preferably implemented as a cascaded pair of emitter follower 
amplifiers using transistors similar to transistors 14 and 30. 
As shown in greater detail in FIG. 2, the optical path 36 includes a series 
of optical elements. A collimating lens 38 is positioned to receive light 
emitted by the laser diode 32. Light passing through the lens 38 is 
directed through a polarizer 40. The polarized light is then directed to 
the beam splitter 34. Preferably, approximately 20% of the light entering 
the splitter 34 is directed to a turning mirror 42 and then through a 
focusing lens 44 to the photodiode 10. The turning mirror 42 permits the 
placement of the photodiode 10 on the same mounting surface as the laser 
diode 32 and minimizes the distance between the diodes. 
It should be noted that the polarizer 40 operates to linearize the optical 
feedback signal carried along the optical path 36. In the absence of the 
polarizer 40, the relative percentages of light passing outward from the 
beam splitter 34 would vary at low light levels, thereby introducing error 
into the system. 
The optical feedback signal passed along the path 36 is indicative of the 
intensity of light emitted by the laser diode 32. Preferably, the length 
of the optical path 36 is limited to avoid introduction of excessive phase 
delay into the feedback loop 
The detailed operation of the circuit shown in FIG. will now be described. 
The multiplying DAC 4 produces a current I.sub.dac at its output which 
represents the product of the received digital data and the multiplier 
value. The digital data and multiplier value may originate from a 
conventional source such as a microprocessor or memory (not shown). The 
digital data may, for example, represent information for forming an image 
on photographic film or other medium, while the multiplier value may 
represent a correction factor for compensating for differences in scanning 
speeds of a scanner (not shown) that sweeps the laser output beam over the 
image medium. 
As the optical feedback signal passed along path 36 impinges on the 
photodiode 10, a current I.sub.d is produced by the photodiode. The 
current I.sub.d is proportional to the intensity of the impinging feedback 
signal. Since the currents entering and leaving the node 5 must equal, the 
difference (if any) between I.sub.dac and I.sub.d is I.sub.e, an error 
current. That is, the magnitude of I.sub.e represents the difference 
between the actual intensity of light emitted by the laser diode 32 and 
the "correct" intensity represented by I.sub.d. The servo 2 operates to 
control the laser diode's output so as to maintain I.sub.e essentially at 
zero. 
More specifically, at low frequencies, the transistor 14 operates as a 
transconductance amplifier of the output of the amplifier 8. The signal 
component of its collector current passes through the resistor 18 and is 
thus converted to a control voltage proportional to the resistance 
R.sub.18, the capacitor 20 having little effect at these frequencies. The 
transistor 14 also operates as an emitter-follower amplifier in the 
feedback path of the amplifier 8, applying the output of the latter 
amplifier to the capacitor 12. The capacitor 12, in turn, provides the 
desired integration of the error signal I.sub.e and produces a control 
voltage which is applied to the buffer amplifier 24. 
Also, the negative feedback around the amplifier 8 forces the voltage at 
the inverting input terminal to zero. This provides zero impedance for the 
current I.sub.d from the photodiode 10. With the low impedance, the 
photodiode 10 operates as a current source, essentially eliminating the 
effect of its junction capacitance and thereby extending the frequency 
range of the photodiode. 
The control voltage is buffered by the buffer amplifier 24, which is 
preferably unity gain, and applied to the transconductance amplifier 26 
(the base of transistor 30). The current flowing through the collector of 
the transistor 30, which is the same as that flowing through the laser 
diode 32 is effectively controlled by the voltage applied to the base of 
transistor 30. 
At high frequencies, the gain of the amplifier 8 diminishes and the 
capacitor 15 presents a very low impedance. The amplifier 8 thus has no 
substantial effect on circuit operations. However, the impedance of the 
capacitor 12 is low, thus providing a low impedance path through this 
capacitor, the base-emitter junction of the transistor 14 and the 
capacitor 15. Again this provides the desired current-source operation of 
the photo-diode 10. Moreover, with this current path, the transistor 14 
operates as a common-base amplifier at these frequencies. The output of 
this amplifier stage passes predominantly through the capacitor 20, which 
serves as an integrator of the error signal, since the voltage across a 
capacitor is the integral of the current through it. 
The crossover frequency for these modes of operation corresponds to the 
time constant R.sub.16 C.sub.12 in the case of the amplifier 8 and the 
time constant R.sub.18 C.sub.20 for the output of the transistor 14. These 
time constants are, therefore, preferably equal and on the order of 50-100 
kHz. In this fashion, the integrating amplifier 7 is able to produce an 
integrated error signal for a wide input frequency range, thus 
substantially increasing the operating bandwidth of the system. 
The foregoing description has been limited to a specific embodiment of this 
invention. It will be apparent, however, that variations and modifications 
may be made to the invention, with the attainment of some or all of the 
advantages of the invention. Therefore, it is the object of the appended 
claims to cover all such variations and modifications as come within the 
true spirit and scope of the invention.