How Our UltraStableLIGHT tm technologies Works

Download White Papers:
An Ultra-Stable VCSEL Light Source (and) Optical Power of VCSELs Stabilized to 30ppm/oC without a TEC

Download Patent: UltraStableLIGHT® US Patent #8,502,452



Laser module built with UltraStableLIGHT TM technologies

Laser module built with UltraStableLIGHTTM technologies.



Overview

The UltraStableLIGHT® technologies enable laser modules to produce 1 mW of optical power from 30 mW of electrical power with unprecedented stability and accuracy without a thermoelectric cooler (TEC). The innovative Optical and Electronic stabilization Methods reduce the temperature coefficient of light-source modules to ±30 ppm/°C. A module made with our technologies (see above image) can run continuously on two coin batteries for eight hours making it ideal for portable sensors and instrumentation as well as wireless health monitoring, environmental data acquisition, and military tasks.


The Stability Problem

The variation of emission wavelength with temperature is an intrinsic attribute of semiconductor light emitters. For VCSELs, it is typically 0.04 to 0.08 nm/C and for laser diodes, the variation ranges from 0.05 to 0.35 nm/C. In semiconductor light sources, the stability problem results from many factors. The major ones include: 1) changes of photodiode responsivity with wavelength and temperature (dS/dλ and dS/dT), 2) variation of responsivity over the active detector area; 3) changes in beam quality (divergence and power profile); 4) polarization drift, and 5) stray and ambient light. Factor 5 includes spontaneous emissions, scattered laser light, and ambient light collected by the monitor photodiode. Stray and ambient light can be virtually eliminated by sound packaging design with apertures and baffles. The remaining factors are nearly linear and have positive and negative slopes that combine to give a net control error, bottom line of Table 1. A stable light source has a small temperature coefficient and vice versa. For example, a light source with a temperature coefficient of 100 ppm per °C is ten times more stable than one having a temperature coefficient of 1,000 ppm °C. The magnitude of major error-causing factors is given in Table 1, which shows that net control error can range from less than -2,000 ppm/°C to more than 6,000 ppm/°C.

Table 1. Contributors to light-source control errors.


Contributors to light-emitter control errors


Optical and Electronic Methods for Stabilizing Optical Power

The relationship among the control factors in our system is expressed in:

The relationship among the control factors in our system

Where: PO is output optical power; TX and R are the transmittance and reflectance of the beamsplitter, ηX is the output coupling coefficient, ηR is the reflection coupling coefficient, S is photodiode responsivity, IREF is a reference current, and PSE, PS and PX are optical power in spontaneous emissions, scattered laser light and ambient light, respectively. The coupling coefficients ηR and ηX incorporate the effects of changing far-field beam structure and quality with temperature. The closed-loop system for implementing this relationship is illustrated in Figure 1.

A laser module built with our Optical Method combines a single-mode VCSEL or LD with a novel beamsplitter (Figure 1) and a proprietary calibration protocol [1,2]. The key element of the optical system is a beamsplitter consisting of a proprietary temperature-compensating beamsplitter (TCB) coating on a fused-silica wedge. The TCB coating reflects more or less light as the emitter wavelength changes with temperature. The rate and polarity of the change in reflectance is set by adjusting the polarization angle of the laser at time of manufacture to precisely compensate for the combined effect of temperature on beam quality, photodetector responsivity, and aging.

System for USL Optical and Electronic power stabilization

Figure 1. System for USL Optical and Electronic power stabilization.



The Electronic Method works with VCSELs, LDs, and resonant-cavity LEDs. The systems combines a light emitter, a beamsplitter comprised of an uncoated wedge (lasers) or plate (LEDs) of high-index glass such as sapphire or LaSF9, a Feedback Controller (Figure 1), and a proprietary calibration protocol [3,4]. The controller is in a feedback loop with the photodiode and APC driver. It adds an error current derived from the temperature signal VT to the monitor photodiode current to generate a control current that is input to the APC driver. The error current can be positive or negative depending in the sign of the net control error. The system is calibrated at time of manufacture.

An example of how well our technologies work is illustrated in Figure 2. The temperature spectrum of an uncompensated light emitter with a temperature coefficient of -2500 ppm/°C is shown on plot A. Plot B shows the temperature spectrum of our TCB coating and plot C illustrates how the temperature-induced variation of relative optical power is virtually eliminated, leaving mainly random noise.


Emitter spectrum (A) is sampled with reflectance spectrum (B); and fed back to a controller to virtually eliminate fluctuations in output power (C).

Figure 2. Emitter spectrum (A), reflectance spectrum (B) and relative output power
(C) for temperature-compensated laser module.



A VCSEL light source made with our UltraStableLIGHT® technologies is shown at the top of this page. A time series of relative optical power emitted from it Figure 3 demonstrates that the RMS value of optical power of the VCSEL light source is only 270 ppm over a 500-hour period with 25°C variations of device temperature.


500 hour record of output power and temperature for an UltraStableLIGHT TM module

Figure 3. 500-hour record of output power and temperature for an UltraStableLIGHTTM module.



1. An Ultra-Stable Laser Light Source.. SPIE-Photonics West 2013, Vertical-Cavity Surface-Emitting Lasers XVII, Conference OE121, John Downing, Dubravko Babic, and Mary Hibbs-Brenner.

2. High-stability light source system and method of manufacturing. United States Patent Number 8,502,452, J. Downing and D. Babic, and other foreign patents

3. Optical power of VCSELs stabilized to 30 ppm/°C without a TEC. SPIE Photonics West 2015 Conference on VCSELs.

4. High-stability light source system and method. United States Patent Application Number 14/546,970, J. Downing (2015).