Abstract:
A light source includes a semiconductor light emitter having an electrical drive input and operatively configured to emit a light beam; a first weakly polarizing beam splitter positioned to capture the light beam, reflecting one portion, and transmitting another portion with an output intensity P. The light source includes a second polarizing beam splitter positioned to capture the reflected portion of the light beam and split it into first and second detector light beams of orthogonal polarizations. The light source further includes first and second detectors capturing those detector light beams, and is configured to deliver corresponding first and second output signals from corresponding detector outputs. The light source includes an electronic circuit coupled to those electrical outputs and to the electrical drive input of the light emitter.

Description:
BACKGROUND 
     Constant-intensity-light sources are ubiquitous in optical sensing systems used in medical, environmental, and industrial applications. Temperature stability of these optical sources is essential as it is the foundation of the instrument accuracy, be it through calibration at manufacturing time or in real time correction. Optical and electrical properties of all light emitting components and optical materials vary with temperature. For this reason an absolutely stable source does not exist. Rather, sources of varying stability are developed for specific applications. In many applications today, where lasers are needed, the control of laser output intensity is realized by stabilizing the laser&#39;s temperature which in turns stabilizes its wavelength and output. Using thermo-electric-coolers (TEC) raises the cost of such laser sources and lowers their wall-plug efficiency. In many applications, for example, actuators for sensors or machine vision, stable intensity and low power consumption are important, while stable emission wavelength is not critical. In this case, uncooled source may be used in combination with sophisticated intensity compensation that makes the source immune to temperature changes over some temperature range and source degradation. Uncooled sources are preferred in systems where size and cost are important. 
     The stabilization of the output beam intensity from lasers and light-emitting diodes is conventionally performed using a closed-loop control system illustrated in  FIG. 1  (PRIOR ART). A light source  101 , which may be an incandescent lamp, a light-emitting diode, or a laser, is powered by control electronic  106  and emits a collimated light beam  105  onto a beam splitter  102 . The transmitted portion of the beam  103  is useful light that may be used for illumination or measurement/sensing. The reflected portion  104  of the incident light  105  is captured by the photodetector  107 . The intensity of the reflected beam  104  is converted to electric current  108  in the photodetector  107 . The control electronics  106  compares the reflected beam  104  intensity in form of current  108  against a reference  110 . 
     The primary factors producing intensity drift in the output beam intensity versus temperature are (a) temperature drift in the reference  110 , (b) the change in the emission wavelength or emission spectra of the light source with temperature, which in turn can change the transmittance to reflectivity ratio of the beam-splitter, and (c) scattered light reaching the detector and offsetting the measured power is also temperature dependent. If the optical source  101  degrades with time, the control electronics  106  will make necessary correction in optical source  101  output intensity  103 . 
     Constant-intensity optical sources based on lasers are known in the field. For example United States patent application 20120025714 by Downing and Babic discloses uses a vertical-cavity surface emitting laser and a weakly polarizing optical interference coating to achieve high stability over temperature and time. The operation of the invention disclosed relies on the temperature drift in the emission wavelength of single-mode linear-polarization-locked lasers. The fine adjustment of the output power temperature drift coefficient is adjusted by rotating the laser around the optical axis in order to adjust the beam&#39;s polarization. An important factor coming to play with a coherent optical source such as a single-mode laser is the appearance of interference fringes appearing on transparent objects external to the source. This is particularly problematic if the beam is used to measure the properties of liquids stored in containers with transparent walls. If interference is a problem in the measurement setup, there is an advantage in using incoherent optical sources. Light-emitting diodes have coherence length which is significantly smaller than the thickness of the walls on most glass pipes and test vials. For this reason, interference fringes rarely occur in the measurement. 
     Light-emitting diodes are used to provide constant light output with reliability, accuracy, and power efficiency that surpass incandescent lamps used in similar instruments historically. However, due to wavelength drift with temperature, high degree of intensity stability (better than 100 ppm/° C.) is not achievable due to the temperature variation in the systems components with wavelength. 
     An additional problem in realizing ultra-stable optical sources is the manufacturing tolerance of components that directly influence the stability response, such as, variation in detector responsivity, manufacturing variation in the interference coating characteristics (if used), or temperature dependence of the comparator at input to the control circuit. 
     This application discloses several embodiments of an optical source based on light-emitting diodes or lasers that feature high temperature stability of the output-beam intensity. 
     One of the objectives of this application is to disclose a highly temperature-stable uncooled optical source of incoherent light. Another objective is to disclose a highly temperature-stable uncooled optical source of coherent polarized light. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This application discloses an improvement on the feedback-loop optical source disclosed in prior art (shown in  FIG. 1 ) by introducing a correction that relies on temperature dependent polarization changes in the optical control loop. A baseline optical source using a light-emitting diode (LED)  100  with a feedback control-loop and a beam splitter is shown in  FIG. 1  (PRIOR ART). A conventional closed-feedback loop optical source shown in  FIG. 1  comprises a light-emitting diode or a laser  101  emitting a beam of light  105  which is incident on a beam-splitter  102 ; a portion of that incident light  105  is transmitted through the beam splitter  102  and becomes useful light beam  103  with controlled intensity, while a portion  104  of the incident beam  105  reflects off the beam-splitter  102  and is captured by a photodetector  107  where it is converted to electrical current  108  and compared to a reference current  110 , the difference is amplified by a differential amplifier  101  and the output from the amplifier is used to drive the current through the light-emitting diode or laser  101 . The beam-splitter may have an interference coating  109  deposited on its incident surface. The nonlinearities in the conversions between the driver  101  and light beam  105  intensity are corrected by the high gain of the control loop, but the temperature dependent changes in the photodetector  107  responsivity and the transmission through the beam splitter  102  are not accounted for. 
     If we denote with P the output intensity  103  of the light source, the output-intensity local temperature sensitivity is dP(T)/dT, defined at every temperature, and output-intensity temperature sensitivity is defined as (P(T 2 )−P(T 1 ))/(T 2 −T 1 ), where T 1  and T 2  are the temperatures at the boundaries of the defined temperature range. When T varies over the temperature range (T 1 -T 2 ), the output-light beam  103  intensity will vary with temperature due to many effects, including the temperature and wavelength drift in the detector  107  responsivity, the drift in the reference source  110  or the comparison between the measured and reference values of current (at the input of the amplifier  101 ), and finally the temperature dependence of the refractive index in the glass or the optional coating  109  of the beam splitter  102 . The wavelength drift occurs because the peak emission wavelength of light-emitting diodes drifts with temperature. For this reason, the temperature variation in the output beam  103  intensity becomes a result of two factors: the temperature performance of the components, but also the change with wavelength of the optical constants of the devices and materials. The wavelength drift is a direct function of the light-emitting diode temperature: these two are deterministically related, but vary from device to device owing to manufacturing variation and have to be experimentally determined for each component individually.  FIG. 2  illustrates the emission spectrum of a commercially available red light-emitting diode measured at several different temperatures. This measurement reveals the key characteristic of light-emitting diodes: As the temperature increases, the output power P reduces with temperature (dP/dT&lt;0) and the peak emission wavelength λ m  increases with temperature (dλ m /dT&gt;0). For lasers, specifically vertical-cavity lasers, the output power may in some cases increase with temperature, but the peak emission wavelength always increases. The typical drift in peak emission wavelength for LEDs is around≈0.1 nm/° C. 
     The first level of output power stabilization of an LED or a laser is to pick off some of the output and feed it to an amplifier to correct the LED drive current as is shown in  FIG. 1 . However, if one measures the peak emission wavelength drift over temperature for the LED (or laser) in use, one can compute the equivalent temperature variation and the required transmission (T) to reflectivity (R) ratio (F=T/R) of the coating  109  on the splitter  102  that would stabilize the output. One then uses this knowledge to design the interference coating  109  so that it produces a change in the output beam intensity  103  with temperature that approximately cancels the temperature drift in the output-beam intensity arising from all other effects in the optical source  100 . We refer to this approach as the second level of stabilization or compensation and it comprises designing an optical coating  109  so that the beam-splitter  102  transmission/reflection ratio F approximately matches the temperature dependence of the baseline system. The difficulty with this second level of compensation is that the interference coating on the beamsplitter has to be identical each time it is manufactured and it would have to track (be adjusted) for every batch of LEDs or lasers that is used in the source. Below we disclose a third level of compensation that resolves this problem by using polarization of the optical coating. 
     In one embodiment of the present invention, temperature compensation of light-source output power is realized by driving an LED in proportion to the degree of polarization of the beam reflected on the weakly polarizing beam splitter—this is referred to as the third-level compensation. The weighting of the reflected beam polarization is done using either a manually or electronically adjustable polarizer or an electronically or manually controllable polarization analyzer. The electronic control is performed using a voltage controlled resistor (a field-effect transistor operated in the linear mode), voltage controlled variable gain amplifier, or by digital control. 
     The beam emitted from the LED is unpolarized and becomes polarized upon reflection on a first weakly-polarizing beam splitter. The polarization state of the reflected beam is resolved by either using an adjustable polarizer, which is disclosed as Embodiment A “Adjustable Polarizer”, or by using two detectors and a second polarizing beam splitter to resolve the beam&#39;s polarization, which is disclosed as Embodiment B “Polarization Analyzer” in the following text. Embodiment B, can also be used with a laser in place of an LED with same structure (referred to as Embodiment C below). 
     For purposes of this application, a beam polarizer is an optical component that transmits (or reflects) a beam of first polarization preferentially and reflects (or transmits) the beam with polarization orthogonal to said first polarization. As is well known in the art, a beam polarizer may be realized as a polarizing beam splitter, as a wire-polarizer, or in other ways. The quality of practical polarizers is expressed in their polarization discrimination ratio (or just polarization ratio) which is the ratio of the transmission (or reflection) intensity coefficients of the two orthogonal polarizations (typically the larger divided by smaller), as is well known in the art. A good polarizer may exhibit a discrimination ratio of 100 or much higher more. A polarizing beam splitter is a polarizer which reflects light of first polarization and transmits light with polarization orthogonal to first polarization. Polarizing beam splitters are available from sources such as Edmunds Optics, Inc. from Barrington, N.J., USA. 
     For the purposes of this application, a weakly polarizing optical coating means a coating that exhibits a polarization ratio of the order of 10 or less when an unpolarized beam is incident on it. This type of coating would be a poor polarizer, but it is useful in the embodiments disclosed in this application. The weakly polarizing reflector may also be a weakly polarizing beam splitter where the reflected and transmitted beams are not parallel. In other words, when an unpolarized beam is incident on a weakly-polarizing reflector or beam splitter, the reflected (or transmitted) light beam will exhibit a degree of polarization less than around 90%. Degree of polarization is defined as the ratio of the intensity of the polarized component to the total intensity of an optical beam. This definition is explained in publicly available texts such as  Statistical Optics  by Joseph W. Goodman, published by John Wiley &amp; Sons, 1985. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Simplified schematic showing a classic optical source stabilized using a feedback control loop (PRIOR ART). 
         FIG. 2 : An example of measured emission spectrum from a powered light-emitting diode at different temperatures. 
         FIG. 3A : (Embodiment A of the optical source using the present invention. 
         FIG. 3B  One embodiment of the optical-source control circuit for Embodiment A optical source. 
         FIG. 4A : Embodiment B of the optical source using the present invention using a light-emitting diode. 
         FIG. 4B : One embodiment of the optical-source control circuit for Embodiments B and C. 
         FIG. 5 : Embodiment C of the optical source using the present invention using a laser. 
         FIG. 6 : Block diagram of functions of embodiment of a subsystem with an optical source made according to Embodiments B or C. 
         FIG. 7 : Block diagram of a calibration system for optical source made according to the present invention. 
         FIG. 8 : (A) and (B) are two embodiments of beam splitters that utilize interference coatings on their surfaces to compensate the temperature dependence of the output power from optical sources made according to Embodiments A, B, or C. 
         FIG. 9A : Exemplary composite functions F and detector responsivities S as a function of temperature. 
         FIG. 9B : Computed function H as a function of temperature. 
         FIG. 9C : Resulting control error. 
         FIG. 10 : Illustration used to explain embodiments BS2, BS3, and BS4. (A) Exemplary composite functions F 1  for incidence surface of one embodiment of the weakly-polarizing beam splitter, (B) Exemplary transmittances T 2  for back surface of one embodiment of the weakly-polarizing beam splitter, and (C) resulting composite functions F for the entire beam splitter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The improvement in the stability of an LED-based optical light source is disclosed in the next two embodiments (A and B). Both embodiments use the following building blocks: a light-emitting diode, optional collimator lens, detector, differential amplifier that drives current through the LED. In normal operation, the optical source is operated at any temperature T within a temperature range T 1  to T 2 . Embodiment C uses the same stabilizing principle, but a single-mode laser to provide the light beam. 
     The light-emitting diode emits a light beam having an emission spectrum E(λ) [W/nm] which exhibits peak emission wavelength λ m . (An example of such a spectrum for different temperatures is shown in  FIG. 2 .) As temperature varies from T 1  to T 2 , the emission spectrum E changes: the peak wavelength shifts towards longer wavelengths approximately linearly (dλ m /dT≈constant) accompanied with some spectrum E(λ,T) broadening. When temperature varies from T 1  to T 2 , the peak emission wavelength varies from λ m1  to λ m2 . In as much as the emission spectrum of LEDs can have a full-width half maximum (FWHM) as large as 20 nm, the wavelength range (λ 1  to λ 2 ) over which the emission spectrum is non-negligible is larger than the extent of emission peak variation, namely, one has λ 1 &lt;λ m1 &lt;λ m2 &lt;λ 2 . We refer to λ m1 -λ m2  as the peak emission wavelength range and we refer to λ 1 -λ 2  as the emission spectrum range. In  FIG. 1 , the emission spectrum range is clearly greater than 60 nm. 
     The photodetectors used in the following embodiments are selected so that they capture the vast majority of the spectrum emitted by the light-emitting diode or laser used as a source. This selection generally refers to the type of material used as the absorbing section of the photodetector pn-junction. Commercial photodetectors are typically packaged into packages with windows for protection. The detectors used in the embodiments may have glass windows, unless the coherence length of the source (LED or laser) is comparable or greater than the thickness of the glass. In this case one may use detectors without a glass window. 
     A differential amplifier is an electronic circuit with two inputs and at least one output, wherein the output is proportional the difference of the signals present at the two inputs. The inputs and output from the amplifier may be current or voltage as is known in the art, but for a specific implementation of the temperature-stable source it may be advantageous to drive the LED with current and accept current as input, since the photodetector generates current when illuminated. The frequency response of the amplifier exhibits DC transfer and an upper cutoff frequency to pass the desired modulation frequency and limit the noise power as is well known in the art of amplifier design. 
     An angle of incidence θ i  for a beam incident on surface is defined as the angle between the incident beam and the direction normal to the surface. The beam and the surface normal define the plane of incidence. 
     I. Embodiment A 
     Adjustable Polarizer 
     One set of embodiments of a temperature-stable incoherent optical source and a method to stabilize an incoherent optical source are illustrated with the help of  FIGS. 3A and 3B . An optical source  300  comprises a light-emitting diode  301 , a beam splitter  302 , a rotatable polarizer  306 , a monitor photodiode  308 , and light-emitting diode driver circuit  312  configured as a differential amplifier with the current output  310  from the photodiode  308  and a reference current input  311  connected to the differential amplifier  312  inputs and the amplifier  312  output driving the light-emitting diode  301 . The light emitted from light-emitting diode  301  is collimated using an optional collimator lens  313  (or lenses) and is incident on beam splitter  302 . The surface of the beam splitter is coated with an interference coating  320 . Since the light-emitting diodes produce a lot of spurious emissions in all directions, a baffle will be needed to encapsulate the light-emitting diode and let only the useful beam  305  out of the encapsulation (not shown in the figures). 
     The intensity of the light  305  incident on beam splitter  302  is denoted with P L . The angle of incidence  314 , denoted θ i , of the beam  305  onto the beam splitter  302  may take any value and is established by design prior to manufacturing. The angle has to be adjusted at optical source manufacturing time so that reflected light is captured by the detector  308 , hence the position of the detector will be adjusted accordingly. The beam splitter  302  is coated with an interference coating  320  on the side that faces the light-emitting diode  301  so that the light beam  305  hits the interference coating  320  first; that side of the beam splitter is referred to as the incidence surface. In one embodiment, the back side, i.e., the other side, of the beam splitter is coated for reduced light reflectivity, i.e., an anti-reflective coating, while in another embodiment the back surface  321  of the beam splitter  302  is positioned at an acute angle relative to the incidence surface  320  so no light reflected on that back surface interferes with the reflected beam  304 : In this case the beam splitter  302  is a prism, as disclosed in later part of the application in connection with  FIGS. 8 a  and 8 b   , rather than a plate as shown in  FIG. 3A . In yet another embodiment the back surface positioned at an acute angle relative to the incidence surface is coated so that the exiting light  303  traverses the back surface at normal incidence and the back surface is coated using an interference coating that exhibits transmission coefficient that varies with wavelength and is designed to assist in the temperature compensation. The coated beam splitter  302  is characterized with optical reflectivity and transmission spectra versus wavelength (and versus temperature, if this is needed) at specific incidence angle θ i  for TE and TM incidence: the transmission (T TE , T TM ) and reflectivity (R TE , R TM ) versus wavelength. The reflectivity is defined as the light power in the reflected beam  304  divided by the power in the incident beam  305 . Transmittivity is defined as the power in the exiting beam  303  divided by the incident beam  305  for each polarization. This definition accounts for the fact that the two surfaces of the beam splitter may be inclined at an angle relative to each other. 
     Composite functions F TE  and F TM  versus wavelength are formed from the ratios of the transmitivity to reflectivity for TE and TM polarization as F TE =T TE /R TE  and F TM =T TM /R TM , respectively. The composite functions F TE  and F TM  characterize the optical characteristics of the beam splitter coated with the interference coating and are generally functions of wavelength, angle of incidence θ i , and temperature T. 
     The output beam  303  is generally weakly polarized and its intensity is given with P OUT =P L ·(T TE  T TM )/2, when the incident beam  305  is unpolarized. A portion  304  of the LED-emitted light  305  is reflected on the coated beam splitter  302 , passes through the rotatable polarizer  306 , and is incident on the monitor photodiode  308 . The beam  305  is generally unpolarized, the beam  304  is weakly polarized (owing to the interference coating  320  on the beam splitter  302 ), and the rotatable polarizer  306  has a high polarization discrimination ratio. The rotatable polarizer  306  is operatively configured to rotate around an axis that is parallel to beam  304  (the rotation is indicated with the arrow  316 ). The amount of rotation of the rotatable polarizer around its axis is expressed in the detector polarization angle φ D . The detector polarization angle φ D  is defined as the angle between the passing polarization direction of the polarizer  306  and the plane of incidence of the beam splitter  302 . The passing polarization direction of a polarizer is the direction of the electric field vector of a linearly polarized light beam for which the polarizer exhibits highest transmittance. Passing polarization direction is perpendicular to the direction of the light beam incident on the polarizer. 
     The differential amplifier  312  exhibits very high gain, so that, according to the theory of operational amplifiers terminated with negative feedback, well known in the art of electronic design, the temperature dependence of the gain and its frequency response have very weak or negligible effect on the overall control ability of the feedback loop. The intensity of the beam received by the detector  308  is converted to current  310 , which is then fed into the inverting input of the differential amplifier  312 . A reference current source  311  is fed into the non-inverting input of the differential amplifier  312 . The amplifier output drives current into the LED. The light-output from the LED P L  is given as approximately proportional to the drive current I LED : P L =I LED /S LED (I LED ). The LED output efficiency is 1/S LED  [W/A], once we close the feedback loop, we can express the output power as, 
     
       
         
           
             
               
                 
                   
                     P 
                     OUT 
                   
                   = 
                   
                     
                       
                         I 
                         REF 
                       
                       
                         
                           
                             S 
                             PD 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   R 
                                   TE 
                                 
                                 ⁢ 
                                 
                                   sin 
                                   2 
                                 
                                 ⁢ 
                                 
                                   ϕ 
                                   D 
                                 
                               
                               + 
                               
                                 
                                   R 
                                   TM 
                                 
                                 ⁢ 
                                 
                                   cos 
                                   2 
                                 
                                 ⁢ 
                                 
                                   ϕ 
                                   D 
                                 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           
                             S 
                             LED 
                           
                           / 
                           G 
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             T 
                             TE 
                           
                           + 
                           
                             T 
                             TM 
                           
                         
                         2 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     All of the elements of the optical source  300  are temperature and wavelength dependent: The reference current I REF , the detector responsivity S PD , the gain G of the amplifier may be low and temperature dependent, and the LED efficiency S LED  which is dependent on the current flowing through the LED due to droop, heating, and coupling losses. Finally, the peak emission wavelength λ m  of the LED moves with temperature with a typical linear coefficient dλ m /dT, all of which make the entire system shown in  300  temperature dependent: P OUT (T, φ D ). 
     In order to compensate for these changes, a second-level compensation comprises in designing an optical interference coating (resulting in specific T and R in equation (1)) that will make the transmittivity and reflectivity on the beam splitter  302  vary exactly in the opposite direction and reduce the temperature variations over temperature. This second-level temperature compensation is possible, but does not lend itself to efficient manufacturing because the component characteristics vary in manufacturing (including the interference coating  320 ) and it becomes next to impossible to obtain the same response every time the optical source is built. In one embodiment, polarizing interference coating  320  is designed so that it produces an output intensity variation with temperature that has opposite signs for the two orthogonal states of rotating polarizer  306 . In another embodiment, a polarization direction (or angle) of rotating polarizer  306  is operatively configured to be adjusted in order to minimize the output intensity variation with temperature. The adjustment may be performed at manufacturing time. In this way, the effect of manufacturing variances is corrected at the last stage of manufacturing, resulting in higher stability optical source. 
     In one embodiment, the coating on the beam splitter  302  is designed so that the product of the temperature coefficient (dP OUT  (φ D =φ 0 )/dT) of the output intensity measured when the polarization angle is set to φ D =φ 0 , and the temperature coefficient (dP OUT  (φ D =φ 0 +π/2)/dT) of the output intensity when the polarization angle is set to φ 0 +π/2 is less than zero. In other words, the output beam intensity temperature coefficients have to have opposite slopes for the two detector polarization angle settings throughout the temperature range. Since the reference for φ D  can be set arbitrarily, we set the reference to zero, i.e., φ 0 =0, and write this condition as: 
                         ⅆ       P   OUT     ⁡     (       ϕ   D     =   0     )           ⅆ   T       ·       ⅆ       P   OUT     ⁡     (       ϕ   D     =     π   /   2       )           ⅆ   T         &lt;   0           (   2   )               
Once the optical source is equipped with this type of interference coating, at the end of the manufacturing, the angle φ D  is adjusted either manually or electronically to make the output beam intensity have a temperature coefficient within some desired range. One can see that this is possible, because in one case the output increases with increasing temperature and on the other it decreases with increasing temperature. This means that the detector polarization angle is adjusted to an optimal value φ D (opt) so that
 
                            ⅆ       P   OUT     ⁡     (       ϕ   D     ⁡     (   opt   )       )           ⅆ   T            =   min           (   3   )               
One embodiment of a control circuit  350  for Embodiment A is shown in  FIG. 3B . The current I LED  flowing through the LED produces unpolarized light that is incident on the beam splitter  302  and then on the polarizing beam splitter  306 . The current I D  is generated by the photodetector D1.  FIG. 3B  shows two amplifier stages delineated with dash-line boxes: differential transimpedance amplifier stage  351  and LED driver stage  352 . The transimpedance amplifier A 1  amplifies the difference between the photodetector current I D  and an externally adjustable reference current I REF . The objective of the differential transimpedance amplifier is to amplify the difference in currents rather than the difference in voltages because the photodetector generates current proportionally to the optical power received. Voltage and current references with high temperature stability are available commercially from, for example, MAXIM Integrated and Linear Technologies, Inc., USA. Clearly, the reference source may be realized by using a voltage reference and precision resistors or other means known in the art, and the current from the photodiode may be converted to voltage at a precision temperature-stable resistor. The output from the stage  351  is voltage V 1  given by V 1 =R 3 I REF −R 1 I D , where R 1  and R 3  are precision resistors. It is essential that the resistors R 1  and R 3  are matched in value and have low temperature coefficients. Precision resistors suitable for this and such purposes are available commercially for use in strain gauges from Vishay Precision Group, Malvern, Pa. 19355, USA; their absolute resistance and temperature coefficients are better than 0.01% and +20 ppm/° C. Using these resistors, the set value of I REF  determines the target optical power in the exit beam  303 .
 
     The output of the differential transimpedance amplifier A 1  is connected to the driver amplifier  352 : The voltage V 1  is amplified to drive the light-source (LED): I LED =G{R 3 I REF −R 1 I D }, where G is the transconductance gain of the last stage  352  which comprises operational amplifier A 3 , resistors R 6  and R 7 , and an optional driver transistor T 3  all configured to control the current I LED  through the light-emitting diode  301 . The driver amplifier circuit  350  shown in  FIG. 3B  is a non-limiting example. It is clear that the differential transimpedance amplifier A1 may be realized as a differential voltage amplifier if the photodetector D1 current I D  is converted to voltage and that voltage is compared to a reference temperature stable voltage source (VREF) rather than a temperature stable current source. 
     Finally, to illustrate the functional dependence of the control and the detector polarization angle, we let the amplifier gain become infinitely large to arrive at the expression for output power as 
     
       
         
           
             
               
                 
                   
                     
                       P 
                       OUT 
                     
                     = 
                     
                       
                         
                           
                             I 
                             REF 
                           
                           2 
                         
                         · 
                         
                           H 
                           ⁡ 
                           
                             ( 
                             
                               ϕ 
                               D 
                             
                             ) 
                           
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       where 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       H 
                       ⁡ 
                       
                         ( 
                         
                           ϕ 
                           D 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         
                           S 
                           PD 
                         
                       
                       · 
                       
                         
                           
                             F 
                             TE 
                           
                           + 
                           
                             F 
                             TM 
                           
                           + 
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               F 
                               TM 
                             
                             ⁢ 
                             
                               F 
                               TE 
                             
                           
                         
                         
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   F 
                                   TE 
                                 
                               
                               ) 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               ϕ 
                               D 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   F 
                                   TM 
                                 
                               
                               ) 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             
                               ϕ 
                               D 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The constraints on the composite functions F TE  and F TM , and the detector responsivity variation with temperature are then given by inserting (4) into (3). 
     II. Embodiment B 
     Electronic Analyzer 
     Embodiments of a temperature-stable incoherent optical source with an electronic analyzer and a method to stabilize an incoherent optical source are disclosed with the help of  FIGS. 4A and 4B . Optical source  400  comprises light-emitting diode  401 , first beam splitter  402 , second beam splitter  406 , two monitor photodiodes  407  and  408 , and light-emitting diode driver circuit  412  configured as a differential amplifier with the weighted current output  410  from photodiodes  407  and  408  and a reference current input  411  connected to the differential amplifier  412  inputs, and the output from amplifier  412  driving light-emitting diode  401 . The light emitted from light-emitting diode  401  is collimated using an optional collimator lens or lenses  413  and is incident on first beam splitter  402 . The surface of the first beam splitter is coated with an interference coating  420 . 
     The intensity of the light  405  incident on the first beam splitter  402  is denoted with P L . The angle of incidence  414  (θ i ) of the beam  405  onto the first beam splitter  402  may take any value and is established by design prior to manufacturing. The first beam splitter  402  is characterized by its reflectivity (R) and transmission (T) spectra versus wavelength (and versus temperature, if needed) at specific incidence angle θ i  for TE and TM: the transmission (T TE , T TM ) and reflectivity (R TE , R TM ) versus wavelength. Transmission (or transmittivity) and reflectivity are defined as above in Embodiment A. The angle of incidence θ i  has to be adjusted at manufacturing time so that reflected light is captured by the detectors  407  and  408 , hence the position of the detectors  407  and  408  and the second beam splitter  406  will be adjusted accordingly. The beam splitter  402  is coated with an interference coating  420  on the side that faces the light-emitting diode  401  so that the light beam  405  hits the interference coating  420  first; that side of the beam splitter is referred to as the incidence surface. In one embodiment, the back side, i.e., the side on which the transmitted beam exits, the beam splitter is coated for reduced light reflectivity, i.e., with an anti-reflective coating, while in another embodiment the back surface of the beam splitter  402  is positioned at an acute angle relative to the incidence surface so no light reflected on that back surface interferes with the reflected beam  404 . In yet another embodiment, the back surface is positioned at an acute angle relative to the incidence surface so that the exiting light  403  traverses the back surface at normal incidence. In another embodiment, the back surface is coated using an interference coating that exhibits transmission coefficient that varies with wavelength and the wavelength-dependent transmission of the coating is designed to assist in the temperature compensation. 
     The composite functions F TE  and F TM  versus wavelength are formed from the ratios of the transmitivity to reflectivity for TE and TM polarization as F TE =T TE /R TE  and F TM =T TM /R TM . The composite functions F TE  and F TM  define the optical characteristics of the first beam splitter coated with the interference coating and are generally functions of wavelength, angle of incidence θ i , and temperature T. 
     Using these definitions, the intensity of output beam  403  is given with P OUT =P L ·(T TE +T TM )/2. The useful output light  403  is the portion of LED-emitted light beam  405  which is transmitted through the first beam splitter  402 , while the rest of the incident beam  405  is reflected on the first beam splitter  402 , and split by the second beam splitter  406  into two beams, each of which is captured by one of the detectors  407  and  408 . The second beam splitter  406  has a high polarization discrimination ratio. The beam  405  is generally unpolarized, the reflected beam  404  is weakly polarized by the first beam splitter  402 , while the two beams incident on the detectors are polarized orthogonally. One of the detectors  407  hence measures the TE polarized component of the reflected beam  404  and converts it to electrical current with responsivity S TE , while the other detector  408  measures the TM component and converts it to current with responsivity S TM . Clearly, what polarization component each detector captures depends on how the second beam splitter  406  is oriented, hence either combination is possible. It is desirable that the responsivities S TE  and S TM  be as close as possible, but in general, due to manufacturing variances they will be slightly different. This will be accounted for in the analysis of the stability of the disclosed optical source presented below. The currents output from the detectors are weighted and then added in element  409 . The weighing factors are expressed as a function of an effective detector polarization angle φ D  as cos 2  φ D  and sin 2  φ D for TM and TE contributions as shown in  FIG. 4A  In  FIG. 4A , the weighting and adding functionality are indicated by labels/elements  417 ,  418 , and  409 , but clearly the actual circuit implementation may vary as is well known in the art. In one embodiment, the unweighted currents from detectors  407  and  408  are first compared with reference  411  and the differences in currents (voltages) are weighted and added. This embodiment will be disclosed in more detail with the help of  FIG. 4B . 
     The differential amplifier  412  exhibits gain G. The current  410  is fed into the inverting input of differential amplifier  412 . A reference current source  411  is fed into the non-inverting input of differential amplifier  412 . The amplifier output drives current into LED  401 . The light-output P L  from the LED is given as approximately proportional to the drive current I LED : P L =I LED I[S LED (I LED )] where the LED output efficiency is 1/S LED  [W/A]. Once we close the feedback loop we can express the output power as, 
     
       
         
           
             
               
                 
                   
                     P 
                     OUT 
                   
                   = 
                   
                     
                       
                         I 
                         REF 
                       
                       
                         
                           
                             
                               
                                 
                                   R 
                                   TE 
                                 
                                 ⁢ 
                                 
                                   S 
                                   TE 
                                 
                                 ⁢ 
                                 
                                   sin 
                                   2 
                                 
                                 ⁢ 
                                 
                                   ϕ 
                                   D 
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   R 
                                   TM 
                                 
                                 ⁢ 
                                 
                                   S 
                                   TM 
                                 
                                 ⁢ 
                                 
                                   cos 
                                   2 
                                 
                                 ⁢ 
                                 
                                   ϕ 
                                   D 
                                 
                               
                               + 
                               
                                 
                                   S 
                                   LED 
                                 
                                 / 
                                 G 
                               
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             T 
                             TE 
                           
                           + 
                           
                             T 
                             TM 
                           
                         
                         2 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     All of the elements of the optical source  400  are temperature and wavelength dependent: The reference current I REF , the amplifier gain G of the amplifier which may be low and temperature dependent, and the LED efficiency which is dependent on the current flowing through the LED due to droop, heating, and coupling losses. Finally, the peak emission wavelength λ m  of the LED moves with temperature, which makes the entire system shown in  400  temperature dependent. 
     In one embodiment, the coating  420  on beam splitter  402  is designed so that the product of the temperature coefficient (dP OUT  (φ D =0)/dT) of the output intensity measured when the polarization angle is zero (φ D =0) and the temperature coefficient (dP OUT (φ D =90°)/dT) of the output intensity when the polarization angle is set to φ D =90° is less than zero. In other words, the output beam intensity temperature coefficients have to have opposite slopes for the two detector polarization angle settings throughout the temperature range. 
                         ⅆ       P   OUT     ⁡     (       ϕ   D     =   0     )           ⅆ   T       ·       ⅆ       P   OUT     ⁡     (       ϕ   D     =     π   /   2       )           ⅆ   t         &lt;   0           (   6   )               
Note that the detector polarization angle is just a number that determines the weighting of the currents from the detectors via the sine and cosine functions.
 
     At manufacturing time, the angle φ D  is adjusted either manually or electronically to make the output beam intensity have minimal or prescribed temperature coefficient. 
     
       
         
           
             
               
                 
                   
                      
                     
                       
                         ⅆ 
                         
                           
                             P 
                             OUT 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 D 
                               
                               ⁡ 
                               
                                 ( 
                                 opt 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                       
                         ⅆ 
                         T 
                       
                     
                      
                   
                   = 
                   min 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     An embodiment of the control circuit for Embodiment B is illustrated in  FIG. 4B . Still referring to  FIG. 4A , the current flowing through the LED produces unpolarized light that is incident on the first beam splitter  402  and then on the polarizing beam splitter  406 . In  FIG. 4B , the current I S  is generated by the photodetector D S  and I P  by the photodetector D P . Furthermore,  FIG. 4B  shows three amplifier stages delineated with dash-line boxes: tandem differential transimpedance (TZ) amplifier stage  451 , dual variable gain amplifier (VGA) stage  452 , and adder and driver stage  453 . The two detectors  407  and  408  from  FIG. 4A  are depicted as reverse biased photodiodes D S  and D P  in  FIG. 4B . The current photogenerated in these two diodes is fed into the inverting inputs of the differential transimpedance amplifiers A S1  and A P1  resulting in voltages V S1  and V P1 . The value of the resistor R 3  should be set to one half of R S2 =R P2 . Each of the transimpedance amplifiers A S1  and A P1  amplify the difference between the photodetector current (I S  or I P ) and an adjustable reference current I REF . The objective is to amplify the difference in currents rather than the difference in voltages because the photodetector generates current proportionally to the optical power received. Temperature-stable voltage and current references with high temperature stability are available commercially from, for example, MAXIM Integrated and Linear Technologies, Inc., USA. Clearly, the reference current source may be realized by using a voltage reference and precision resistors or other means known in the art. The two outputs from the stage  451  are voltages V S1  and V P1  given by
 
 V   S1   =R   3   I   REF   −R   S1   −R   S1   I   S    V   P1   =R   3   I   REF   −R   P1   I   S   (8)
 
In the stage  452 , the two voltages V S1  and V P1  are amplified independently and with controllable amount of gain by amplifiers A S2  and A P2 , having total voltage gain A S  and A P . The key feature of the second stage ( 452 ) is that the ratio of the gains A S /A P  is controllable. This adjustment may be accomplished using dual gang potentiometers in the feedback of the amplifiers (not shown) where one potentiometer increases while the other decreases the resistance and thereby changing the gain of the amplifier or by controlling the voltage on the shown field-effect transistors T 1S  and T 1P  operated in the linear mode in such a way that the one increases and the other decreases the amplifier gain simultaneously. The voltages at the two outputs from stage  452  are then given by
 
 V   S2   =A   S ( R   3   I   REF   −R   S1   I   S ) V   P2   =A   P ( R   3   I   REF   −R   P1   I   S )  (9)
 
Finally, these two voltages are added and amplified in stage  453  to drive the light-source (LED)  454 .
 
 I   LED   =G{A   P ( R   3   I   REF   −R   P1   I   P )+ A   S ( R   3   I   REF   −R   S1   I   S )}  (10)
 
where G is the transconductance gain of the last stage  453 . The resistors R 6S , R 6P , R 7 , and transistor T 3  are shown as an exemplary implementation of the last stage. It is essential that the resistors R S1 , R P1 , and R 3  are matched in value and have low temperature coefficient. In this way, it is the set value of I REF  that determines the target output power. Making that assumption, the output current comes out to be
 
                       I   LED       GR   3       =         (       A   P     +     A   S       )     ⁢     I   REF       -       A   P     ⁢     I   P       -       A   S     ⁢     I   S                 (   11   )               
The light emitted by LED  401  is passed through weakly polarizing interference coating  420 . The received light on the detectors generates currents given by
 
                     I   P     =             S   TE     ⁢     R   TE         S   LED       ⁢     I   LED     ⁢           ⁢     I   M       =           S   TM     ⁢     R   TM         S   LED       ⁢     I   LED                 (     12   ⁢   A     )                 P   OUT     =         I   LED       2   ⁢           ⁢     S   LED         ⁢     (       T   TE     +     T   TM       )               (     12   ⁢   B     )               
Combining these relationships we finally have the expression for output power as
 
                     P   OUT     =           T   TE     +     T   TM       2     ⁢       I   REF                 S   LED       GR   3       +         A   P         A   P     +     A   S         ⁢     S   TM     ⁢     R   TM       +                   A   S         A   P     +     A   S         ⁢     S   TE     ⁢     R   TE                         (   13   )               
Clearly, if we make the transconductance gain very large (G&gt;&gt;1), we can obtain a relationship for the output power as
 
                       P   OUT     =         I   REF     2     ·     H   ⁡     (     ϕ   D     )           ⁢           ⁢     H   ⁡     (     ϕ   D     )       =         F   TE     +     F   TM     +     2   ⁢           ⁢     F   TM     ⁢     F   TE               (     1   +     F   TE       )     ⁢     S   TM     ⁢     sin   2     ⁢     ϕ   D       +       (     1   +     F   TM       )     ⁢     S   TE     ⁢     cos   2     ⁢     ϕ   D                   (   14   )               
Here A P /A S =tan 2  φ D . Clearly, by varying A P  or A S  and alternatively setting them to zero, we can access all detector polarization angles 0≦φ D ≦π/2. Therefore, the circuit shown in  FIG. 4B  can be used to implement the temperature compensation scheme disclosed in embodiment B. It is clear that this is only one possible implementation of the control scheme for embodiment B, and that other circuit configurations, for example using reference voltage source rather than a current source and changing the tandem differential TZ amplifier accordingly, are possible. Additionally, one could use a digital control loop without departing from the spirit of the invention.
 
     III. Embodiment C 
     Laser or Polarized Source with Electronic Analyzer 
     Embodiments of a temperature-stable polarized optical source and a method to stabilize a polarized optical source are disclosed with the help of  FIG. 5 .  FIG. 5  is identical to  FIG. 4A , with the exception that it uses a laser  501  rather than a light-emitting diode  401 . In one embodiment, the laser may be replaced with a light-emitting diode and a polarizer in front of it so the light emitted by the light-emitting-diode-polarizer assembly is polarized. In either case, the light reaching beam splitter  502  is polarized and the mathematical treatment shown below applies to both cases. 
     An optical source  500  comprises of a linearly polarized laser  501 , first beam splitter  502 , second beam splitter  506 , two monitor photodiodes  507  and  508 , and laser driver circuit  512  configured as a differential amplifier with the weighted current output  510  from photodiodes  507  and  508  and a reference current input  511  connected to the inputs of differential amplifier  512  and the output of amplifier  512  driving laser  501 . The light emitted from laser  501  is collimated using an optional collimator lens  513  and is incident on first beam splitter  502 . The surface of the first beam splitter is coated with an interference coating  520 , this surface is also referred to as the incidence surface. The plane of incidence of the beam splitter  502  is defined as the plane that is parallel to the light beam  505  emitted from the laser  501  and perpendicular to the incidence surface of the beam splitter  502 . 
     The light beam  505  emitted from the laser  501  is linearly polarized with the electric field vector closing an angle φ L  with the plane of incidence of first beam splitter  502 . The intensity of the light beam  505  incident on the first beam splitter  502  is denoted with P L . The angle of incidence  514  (Of) of the light beam  505  onto the first beam splitter  502  may take any value and is established by design prior to manufacturing. The first beam splitter  502  is characterized by its reflectivity (R) and transmission (T) spectra versus wavelength (and versus temperature, if needed) at specific incidence angle θ i  for TE and TM: the transmission (T TE , T TM ) and reflectivity (R TE , R TM ) versus wavelength. Transmittivity and reflectivity are defined as above in the section on Embodiment A. Composite functions F TE  and F TM  versus wavelength are formed from the ratios of the transmitivity to reflectivity for TE and TM polarization as F TE =T TE /R TE  and F TM =T TM /R TM . The composite functions F TE  and F TM  define the optical characteristics of the first beam splitter coated with the interference coating and are generally functions of wavelength, angle of incidence θ i , and temperature T. 
     Using these definitions, the output beam  503  intensity is given with P OUT =P L (T TE  sin 2  φ L +T TM  cos 2  φ L )/2. A portion  503  of the laser-emitted light beam  505  is transmitted through the first beam splitter and becomes useful output light  503 , while the rest of the incident beam  505  is reflected on the first beam splitter  502 , and split by the second beam splitter  506  into two beams, each of which is captured by one of the detectors  507  and  508 . The second beam splitter  506  has a high polarization discrimination ratio. The beam  505  is linearly polarized, the reflected beam  504  is generally elliptically polarized by the first beam splitter  502 , while the two beams incident on the detectors are polarized orthogonally. One of the detectors  507  hence measures the TE polarized component of the reflected beam  504  and converts it to electrical current with responsivity S TE , while the other detector  508  measures the TM component and converts it to current with responsivity S TM . Clearly, what component each detector captures depends on how the second beam splitter  506  is oriented, hence either combination is possible. It is desirable that the responsivities S TE  and S TM  be as close as possible, but in general, due to manufacturing variances they will be slightly different. This will be accounted for in the analysis of the stability of disclosed optical source in later text. The currents output from the detectors are weighted and then added in element  509 . The weighing factors are expressed as a function of an effective detector polarization angle φ D  as cos 2  φ D  and sin 2  φ D  for TM and TE contributions. The weighting and adding functionality are shown with elements/labels  517 ,  518 , and  509  in  FIG. 5  are illustrated as functions, but clearly their actual circuit implementation may vary. For example, the adding can be implemented as a part of the differential amplifier circuit similar to the one shown in  FIG. 4B , while the weighting may be implemented using adjustable potentiometers, using field-effect transistors operated in linear mode, or using digital electronics where the currents from the detectors first have to converted to digital format and then processed in a microprocessor. The output beam intensity becomes, 
     
       
         
           
             
               
                 
                   
                     P 
                     OUT 
                   
                   = 
                   
                     
                       
                         I 
                         REF 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               T 
                               TE 
                             
                             ⁢ 
                             
                               sin 
                               2 
                             
                             ⁢ 
                             
                               ϕ 
                               L 
                             
                           
                           + 
                           
                             
                               T 
                               TM 
                             
                             ⁢ 
                             
                               cos 
                               2 
                             
                             ⁢ 
                             
                               ϕ 
                               L 
                             
                           
                         
                         ) 
                       
                     
                     
                       
                         
                           
                             
                               
                                 R 
                                 TE 
                               
                               ⁢ 
                               
                                 S 
                                 TE 
                               
                               ⁢ 
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 ϕ 
                                 L 
                               
                               ⁢ 
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               
                                 ϕ 
                                 D 
                               
                             
                             + 
                           
                         
                       
                       
                         
                           
                             
                               
                                 R 
                                 TM 
                               
                               ⁢ 
                               
                                 S 
                                 TM 
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               
                                 ϕ 
                                 L 
                               
                               ⁢ 
                               
                                 cos 
                                 2 
                               
                               ⁢ 
                               
                                 ϕ 
                                 D 
                               
                             
                             + 
                             
                               
                                 S 
                                 LED 
                               
                               / 
                               G 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     The circuit shown in  FIG. 4B  is a non-limiting example of a circuit that can also be used to control the optical source of Embodiment C. Embodiment C which uses a laser assumes that the laser will be mounted relative to the first beam splitter incidence plane at manufacturing time to a specific angle. Clearly, if this angle is 45°, then mathematically, relationship (9) will allow functionally the same advantages as the control of Embodiment B, since cos 2  φ L =sin 2 φ L . However, it is visible from equation (15) that embodiment C offers one additional advantage over embodiment B: The laser angle φ L  may be designed to weigh the importance of TE versus TM properties of the first beam coating  520 . This weighting is visible in equation (15) by the appearance of products R TE  sin 2  φ L  and R TE  cos 2 φ L . This specific advantage becomes very useful when the ratio of the TE and TM transmission (and reflection) on the beam-splitter coating needs to be adjusted in magnitude. Note, however, that setting the laser angle φ L  to zero or 90° removes the possibility of using polarization to track the changes in temperature, namely, the photodetector angle φ D  cannot be used to compensate for the temperature dependence of the optical source intensity. 
     All of the elements of optical source  500  are temperature and wavelength dependent: reference current I REF , the amplifier gain G which may be low and temperature dependent, and the laser efficiency which is dependent on the current flowing through the laser due to heating and coupling losses. Finally, the peak emission wavelength λ m  drift and the polarization drift of the laser move with temperature, which makes the entire system shown in  500  temperature dependent. 
     In one embodiment, the coating  520  on the beam splitter  502  is designed so that the product of the temperature coefficient of the output intensity measured when the polarization angle is zero dP OUT  (φ D =0)/dT and the temperature coefficient of the output intensity when the photodetector polarization angle φ D  is set to 90 degrees dP OUT  (φ D =90°)/dT is less than zero. In other words, the output beam intensity temperature coefficients have to have opposite slopes for the two detector polarization angle settings throughout the temperature range. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ⅆ 
                         
                           
                             P 
                             OUT 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 D 
                               
                               = 
                               0 
                             
                             ) 
                           
                         
                       
                       
                         ⅆ 
                         T 
                       
                     
                     · 
                     
                       
                         ⅆ 
                         
                           
                             P 
                             OUT 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 D 
                               
                               = 
                               
                                 π 
                                 / 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                       
                         ⅆ 
                         T 
                       
                     
                   
                   &lt; 
                   0 
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     At manufacturing time, the angle φ D  is adjusted either manually or electronically to make the output beam intensity have a temperature coefficient within some desired range. 
                            ⅆ       P   OUT     ⁡     (       ϕ   D     ⁡     (   opt   )       )           ⅆ   T            =   min           (   17   )               
The circuit shown in  FIG. 4B  also represents the Embodiment C. The current flowing through the laser I LD  (in  FIG. 4B ) produces linearly polarized light that is incident on the beam splitter and is then split on the polarizing beam splitter. The current I S  is generated by the TE photodetector and I P  by the TM photodetector.  FIG. 4B  shows three amplifier stages delineated with dash-line boxes: tandem differential transimpedance amplifier stage  451 , tandem variable gain amplifier stage  452 , and adder and driver stage  453 . Each of the transimpedance amplifiers A S1  and A P1  amplify the difference between the photodetector current (I S  or I P ) and an adjustable reference current I REF . The objective is to amplify the difference in currents rather than the difference in voltages because the photodetector generates current proportionally to the optical power received. Temperature-stable voltage and current references with high temperature stability are available commercially from, for example, MAXIM Integrated and Linear Technologies, Inc., USA. Clearly, the reference current source may be realized by using a voltage reference and precision resistors or other means known in the art. The two outputs from stage  451  are voltages V S1  and V P1  given by
 
 V   S1   =R   3   I   REF   −R   S1   I   S    V   P1   =R   3   I   REF   −R   P1   I   S   (18)
 
     In the stage  452 , the two voltages V S1  and V P1  are amplified independently and with controllable amount of gain: Amplifier A S2  and A P2  having total voltage gain A S  and A P . The key feature of the second stage ( 452 ) is that the ratio of the gains A S /A P  is controllable. This may be accomplished using dual gang potentiometers in the feedback of the amplifiers where one potentiometer increases while the other decreases the resistance and thereby changing the gain of the amplifier or by controlling the voltage on the shown field-effect transistors T 1S  and T 1P  operated in the linear mode in such a way the one increases and the other decreases the amplifier gain simultaneously. The voltages at the two outputs from the stage  452  are then given by
 
 V   S2   =A   S ( R   3   I   REF   −R   S1   I   S ) V   P2   =A   P ( R   3   I   REF   −R   P1   I   S )  (19)
 
Finally, the two voltages are added and amplified in stage  453  to drive the laser diode (LD)  454 .
 
 I   LD   =G{A   P ( R   3   I   REF   −R   P1   I   P )+ A   S ( R   3   I   REF   −R   S1   I   S )}  (20)
 
where G is the transconductance gain of the last stage  453 . It is essential that the resistors R S1 , R P1 , and R 3  are matched in value and have low temperature coefficient. In this way, it is the set value of I REF  that determines the target output power. Making that assumption, the output current comes out to be
 
                       I   LD       GR   3       =         (       A   P     +     A   S       )     ⁢     I   REF       -       A   P     ⁢     I   P       -       A   S     ⁢     I   S                 (   21   )               
The light emitted by laser  401  is passed through weakly polarizing interference coating  520 . The received light on the detectors is then given by
 
                     I   P     =             S   TE     ⁢     R   TE         S   LD       ⁢     I   LD     ⁢     cos   2     ⁢     ϕ   L     ⁢           ⁢     I   M       =           S   TM     ⁢     R   TM         S   LD       ⁢     I   LD     ⁢     sin   2     ⁢     ϕ   L                 (     22   ⁢   A     )                 P   OUT     =         I   LD       2   ⁢           ⁢     S   LD         ⁢     (         T   TE     ⁢     cos   2     ⁢     ϕ   L       +       T   TM     ⁢     sin   2     ⁢     ϕ   L         )               (     22   ⁢   B     )               
Combining these relationships we finally have the expression for output power as
 
                     P   OUT     =       I   REF     ⁢           T   TE     ⁢     cos   2     ⁢     ϕ   L       +       T   TM     ⁢     sin   2     ⁢     ϕ   L                     S   LD       GR   3       +         A   P         A   P     +     A   S         ⁢     S   TM     ⁢     R   TM     ⁢     sin   2     ⁢     ϕ   L       +                   A   S         A   P     +     A   S         ⁢     S   TE     ⁢     R   TE     ⁢     cos   2     ⁢     ϕ   L                         (   23   )               
Clearly, if we make the transconductance gain very large (G&gt;&gt;1), we can obtain a straightforward relationship for the output power as
 
 P   OUT   =I   REF   H (φ L ,φ D )  (24)
 
where
 
               H   ⁡     (       ϕ   L     ,     ϕ   D       )       =     1                 S   TM     ⁢     sin   2     ⁢     ϕ   D           F   TM     +         F   TE         tan   2     ⁢     ϕ   L         ⁢     (       1   +     F   TM         1   +     F   TE         )           +                   S   TE     ⁢     cos   2     ⁢     ϕ   D           F   TE     +       F   TM     ⁢     tan   2     ⁢       ϕ   L     ⁡     (       1   +     F   TE         1   +     F   TM         )                           
Here A P /A S =tan 2  φ D . Clearly, by varying A P  or A S  and alternatively setting them to zero, we can access all detector polarization angles 0≦φ D ≦π/2. This can be done at the end of the manufacturing electronically. Setting of the laser angle 0&lt;φ L &lt;π/2 is done by design.
 
     It is clear that this is only one possible implementation of the control scheme for embodiment C, and that other circuit configurations, for example, using a reference voltage source rather than a current source, and using a digital control loop are also possible without departing from the spirit of the invention. 
     IV. Interference Coating on First Beam Splitter 
     The principle of temperature stabilization of a light source disclosed in Embodiments A, B, and C, will be illustrated with the help of  FIGS. 4A, 9A, 9B, and 9C , for the case when the light emitter is unpolarized (Embodiment B), but the design principles for A and C are basically the same. 
     The interference coating  420  is designed for specified angle of incidence  414 . The composite functions F TE  and F TM  for this hypothetical coating are illustrated in  FIG. 9A  along with hypothetical temperature variation of the detector responsivities (S TE  and S TM ) for the two detectors are shown versus wavelength in  FIG. 9A . Note that the slopes are different on all the curves. In order to predict the output power variation with temperature, we need to convert the wavelength variation to temperature variation, and account for the fact that the detector responsivities are also temperature dependent. The resulting function H(T,φ D ) is illustrated in  FIG. 9B  for values of φ D =0, 15, 30, 45, 60, 75, and 90 degrees. The extreme values are for φ D =0 and 90 degrees. A S  one varies the detector polarization angle φ D  and computes the optical source temperature stability error, one can see in  FIG. 9C  that there is a value for which the error is at a minimum. In one embodiment, this is the global optimal detector polarization angle one sets and locks for the life of the optical source. 
     V. Beam Splitter Designs 
     Embodiment BS1 
     As shown in  FIGS. 3A, 4A, and 5 , in one embodiment the weakly polarizing beam splitters  302 ,  402 , and  502  are realized as single-plate polarizers coated on one side with an interference coating  320 ,  420 , or  520 . In other embodiments, both sides of the plates  302 ,  402 ,  502  are coated with interference coatings: The incident surfaces  320 ,  420 ,  520  are weakly polarizing, while the back (opposite) surfaces of the plate are coated with an anti-reflective coating. In yet another embodiment, the back surface is coated with another weakly polarizing coating so that the coating on the front and the back of the slab beam-splitter is combined, and the desired wavelength dependence of R and T is achieved. 
     Embodiment BS2 
     In one embodiment, illustrated with the help of  FIG. 8A , the first beam splitter  804  is a prism where the incidence surface  820  and the back surface  821  are coated with different weakly-polarizing interference coatings, both designed for oblique incidence, but with different wavelength and polarizing properties. The light  803  coming from the light-emitting diode or laser is reflected on incident surface  820  and directed to beam splitter  814  that discriminates the polarization into two orthogonally polarized beams  806  and  807  as described previously in  FIG. 4A . The coating  820  on the incidence surface  820  may be designed with one polarization discrimination and one type of wavelength dependence, while the coating  821  on the back surface may be made with second polarization and wavelength dependence that is different from the first one. This embodiment (using two more than one interference coating) is further explained with the help of  FIG. 10  and  FIG. 8A , where  FIG. 10A  shows exemplary composite functions F TE  and F TM  versus wavelength for the coating  820  disposed on the incidence surface of beam splitter  804  or equivalent.  FIG. 10B  shows exemplary transmittances T TE  and T TM  versus wavelength of the coating  821  disposed on the back surface of the beam splitter  804  or equivalent. It is clear that the composite functions of the coating  820  on the incidence surface exhibit different slopes in the wavelength range of interest, but both slopes are positive. If one were to use the coating shown in  FIG. 10A  by itself in Embodiments A, B, or C, the temperature dependence of the output light intensity would generally not satisfy the requirement described with equation (2) (same as equation (6) and (16)). However, the coating  821  designed for the back surface of the beam splitter exhibits negative slopes in the transmittance spectra and also exhibit relatively weak polarization dependence. It is straightforward to show that the composite functions of the entire beam splitter  804  (or equivalent) include the effects of both coatings ( FIG. 10A  and  FIG. 10B ) via,
 
 F   TE   =F   1TE   F   2TE    F   TM   =R   1TM   T   2TM   (25)
 
where the subscript 1 refers to the incidence surface and subscript 2 to the back surface. The composite functions of the entire beam-splitter are shown illustratively in  FIG. 10C  and they clearly satisfy the optimal condition given with equations (2), (6), or (16). It is clear from expressions (25) that using two coatings (different on the incidence and on the back surface) the design of composite functions for the entire beam-splitter has acquired another degree of freedom. This is particularly important when the specific wavelength and temperature needs are such that obtaining the desired composite function slopes is difficult with one coating design. This embodiment allows the designer to use two simpler designs to accomplish an otherwise difficult wavelength-dependent characteristic. Namely, this embodiment has the advantage of independent control of reflection and transmission properties of the prism. Note that in  FIG. 10 , wavelength dependence translates to temperature dependence when an optical source with temperature dependent peak emission wavelength is used. Hence, the graphs may be viewed as having either wavelength or temperature on the horizontal axis.
 
     Embodiment BS3 
     In another embodiment, the back surface of element  804  is designed for normal incidence so that the coating  821  on the back surface is non-polarizing and alters the transmission versus wavelength characteristics equally for both polarizations. This embodiment is not shown exactly in  FIG. 10B , but can be envisioned if one imagines that the transmission characteristic of the back surface has no polarization dependence, namely, T 2TE =T 2TM  in  FIG. 10B . In this way, only the incident surface coating provides weak polarization discrimination, while the second surface makes global temperature drift compensation. In the end, the coatings accomplish the desired temperature compensation properties given with equations (2), (6) or (16). 
     Embodiment BS4 
     In one embodiment, the first and the second beam splitters of  FIGS. 4A and 5  are merged into one prism. This embodiment is explained with the help of  FIG. 8B . Three-way beam splitter  850  performs two functions: it reflects a portion of the collimated beam  851  coming from a laser or a light-emitting diode as it impinges on the first surface  857  of the beam splitter  850  resulting in beam  860 , it transmits the rest of the beam  852  and splits that beam into two beams on surface  856 : the transmitted beam  853  and reflected beam  854  having orthogonal polarizations. The two beams  853  and  854  are captured by detectors  873  and  874 , respectively. Besides reducing the number of optical components in the system, this embodiment allows each of the three sides of the beam splitter to be coated with a different optical interference coating. For example, a weakly polarizing coating  861  may be used at the first surface  857 , a strongly polarizing coating  863  used at the second surface  856 , and a non-polarizing coating  864  with a global wavelength dependence used at the third surface  855 . The angles of the three-way prism can be designed to any values that suit the designer of the coatings and the mechanics of the optical source. In one embodiment, illustrated in  FIG. 8B , one of the angles of the prism is a right angle and the smaller of the two other angles is adjusted so that two beams  853  and  854  exit the prism at right angles and one of the beams  854  exits normally from the surface  855 . This would make the mechanical position of the detectors simpler and would ensure that the coating  864  on the third surface  855  is non-polarizing. The conditions for a right-angle design are given with the equation  878  in  FIG. 8B , where the angles are also indicated. The refractive index of the prism is denoted with n. 
     VI. Method of Calibrating the Optical Source 
     The method of calibrating the optical source (at the manufacturing stage) disclosed in Embodiments A, B, and C is explained with the help of  FIG. 7 . In step 1, the optical source  713  is assembled and mounted onto a temperature controlled stage or oven  710 . The optical source  713  is turned on and its output beam  714  is captured and measured using a standard calibrated photodetector  711  that preferably remains at constant temperature unrelated to the stage or the oven  710 . The temperature  717  of the oven or the stage (“Tamb”) is controlled using a calibration controller  712  which is essentially a computer or a microprocessor. The calibration controller  712  monitors the temperature of the oven or hotplate via link  716 , makes readings of detected output power  714  via a link  715  to the calibrated photodetector  711 , and controls the detector polarization angle within the optical source  713 . In step 2, the temperature  717  of the oven  710  is gradually swept over the temperature range (from T 1  to T 2 ), and while the temperature is gradually increased through every temperature of interest, the detector polarization angle of the optical source  713 , subject to control by the calibration controller  712 , is varied from zero to 90°, and the output power  714  is noted. The power may be noted just for the two extreme values of the detector polarization angle (zero or 90°) or for a larger selection of angles in between and including the extreme values. The output power will approximately follow the relationship shown with equation (13) and the collected data may be used to fit the unknown variables of equation (13). In one embodiment of step 3, the data is used to determine the single detector polarization angle for which the output power (intensity) variation is at minimum. The selected detector polarization angle is referred to as the “global optimal detector polarization angle” φ D (opt). The computed global optimal detector angle is stored in the memory of the optical source  713  by either (i) turning and fixing a potentiometer inside the optical source  713 , or by (ii) storing the angle value in a computer memory inside the optical source  713  and using a microprocessor or any other means known in the art to hold the voltage fixed on the dual variable gain amplifiers. The purpose of both approaches (i) and (ii) is to hold fixed the ratio of A P /A S . In another embodiment of step 3, a specific value of output power is selected and an optimal φ D  is found during the calibration procedure for every temperature so that the overall output power remains constant. In this embodiment, the output power variation is further minimized relative to the stability achievable with setting only the global detector polarization angle. 
     VII. Entire Optical-Source Subsystem 
       FIG. 6  illustrates the control and data flow inside a self-contained subsystem that emits temperature stable light and allows external remote control. This subsystem embodiment employs an optical source made according to embodiments B, or C, or their equivalents. 
     The subsystem comprises light-emitting diode or laser  670  which emits a beam of light that is partially reflected  677  according to the embodiments B or C and captured and detected by two detectors  671 , and partially emitted from the subsystem as useful light beam  676 . The detected light intensity  677  is converted to current at the photodetectors  671  and the tandem differential transimpedance amplifier  651  (in  FIG. 4B , this is dash-line box  451 ), the two voltage outputs from  651  are amplified in the dual VGA stage  652  and fed to the adder/driver circuit  653  to drive the light-emitter  670 . The ratio of the two gains A P /A S  in the dual VGA stage is controlled externally using a one or two voltage lines provided by a D/A converter  655 . A thermometer  656  (typically a thermistor or a thermocouple) is used to measure the temperature of the optical source housing, namely the ambient temperature. This temperature information is converted to digital information and interpreted by the microprocessor  660 . Based on the temperature value read by the thermometer  656 , the microprocessor  660  computes the memory location in the EPROM  657  it needs to read and send to the Dual VGA stage  652  The sent value is the digital equivalent of the detector-polarization angle read from a table stored in EPROM  657 . The data from the table stored in EPROM  657  is used by microprocessor  660  to determine the detector polarization angle associated with the measured temperature in case that the detector polarization angle is supposed to change with temperature or reads a fixed value in the case that the detector polarization angle is to remain constant throughout the operation of the source. The value of the angle is converted into a digital setting to be sent to DAC  655 , then converted to an analog voltage (or voltages) and in this way the gain ratio in the Dual VGA is set to the required value. Under normal operation of optical source  650 , this sequence is repeated any number of times per second limited by the speed of the microprocessor and the memory reading. 
     At the manufacturing stage, optical source  650  has to be calibrated. An example of the calibration system has already been disclosed in connection with  FIG. 7  in the above text. The calibration is initiated externally by sending a signal to microprocessor  660  via an external interface  659 , such as, USB protocol, over electrical communication lines  678 . 
     VIII. Other Variations on the Embodiments 
     It will be clear to a person of ordinary skill in the art that a number of variations on the architectures disclosed in the figures and the written description can be made without departing from the spirit of the invention. For example, the useful output beam ( 403  in  FIG. 4A ) was taken as the beam transmitted through the beam splitter, while the beam used to monitor the power was reflected. It is clear that these two can be exchanged so that the beam exiting the optical source is actually reflected from the beam splitter and the beam transmitted is use to monitor the optical source power. One reason to employ this change would be to invert the composite functions F TE  and F TM  if they are undesirably large (or small). In yet another embodiment, the disclosed optical source provides optical output intensity that varies linearly with temperature, or varies with any custom function of temperature, rather than simply having a minimal variation with temperature.