Final Report: Robust Diode Lasers for Monitoring and Measurement Technologies

EPA Contract Number: 68D03010
Title: Robust Diode Lasers for Monitoring and Measurement Technologies
Investigators: Anderson, Michael H.
Small Business: Vescent Photonics, Inc.
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: April 1, 2003 through September 30, 2003
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2003) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)

Description:

The purpose of this Phase I research project was to determine the feasibility of building a compact, external cavity diode laser based on a new giant electro-optic (EO) effect. This EO technology can be used to tune a wavelength-selective element and the external cavity path length, enabling mode-hop-free tuning over selectable ranges within the overall tuning range. The goal was to design an external cavity that could enable the tuning of the laser diode gain curve in 5-nm sections. These lasers are needed for compact, robust, spectroscopy-based trace-gas sensors that can monitor vehicle emissions and combustion processes, detect volatile organic compounds, and potentially detect explosives and chemical warfare agents. The new laser has been called the WECSL.

Vescent Photonics, Inc.'s objectives were to demonstrate:

· A suitably large EO effect, with sufficient transmission clarity for use in an external cavity diode laser.

· Stable EO control, free from any temporal perturbations.

· An external cavity diode laser design, complete with rate equation modeling for PI characteristics, side-mode-suppression ratio, and linewidth.

Summary/Accomplishments (Outputs/Outcomes):

Vescent Photonics, Inc.'s modeling efforts reached the conclusion that with suitable designs, operational voltages were achieved under 50 V. The experimental data demonstrated a very large EO effect and absorption losses were less than 1.5 dB. It also was discovered that the effect was stable and that temporal transients, which could have led to undesirable phase and frequency modulation on the laser diode, could be avoided with proper design.

Other loss mechanisms also were modeled, such as coupling between the laser diode and the external cavity. Coupling efficiencies between 70 and 95 percent were found.

Rate equation models were developed for the laser diode and coupled to the external cavity using the experimentally measured parameters for the EO effect and the calculated coupling losses. The laser threshold current increased from 15 mA for the bare diode to approximately 20 mA for the diode coupled to the cavity. The slope efficiency typically was 75 percent of that for the bare diode, and 50 percent in the worst case. Hence, the power output from the WECSL will be about one-half that of the bare diode. The dispersion of the wavelength-selective element was computed, and the side-mode suppression ratio of the WECSL was estimated. The linewidth also was computed, which, as expected, was greatly reduced because of the extended volume of the cavity.

Two designs for the WECSL were developed. The magnitude of the EO effect demonstrated in this work ensures that the first design will tune 93 nm in the 1.55 µm range in 3 nm mode-hop-free sections that can be electronically placed anywhere within the tuning range. The second laser is a less expensive and simpler version that can tune 11 nm in 3 nm mode-hop-free sections. The center wavelength of the short-range WECSL can be placed anywhere within the gain profile of the diode laser, making it an attractive alternative to hard-to-get distributed feedback (DFB) lasers. Table 1 shows the anticipated specifications of the WECSL.

Table 1. Anticipated specifications of the WECSL (values for IR operation, 1.3 µm to 2.0 µm)

Specification
Short-range WECSL
Long-range WECSL
Total tuning range 11 nm 94 nm
Mode-hop-free tuning range* 3.3 nm 3 nm
Power output 7 mW** 5 mW**
Threshold current 20 mA*** 25 mA***
SSR > 40 dB > 40 dB
Linewidth < 100 kHz < 100 kHz
Size
     Laser submount 1 cm x 1 cm 1 cm x 2.5 cm
     Final package 1.5 cm x 1.5 cm 1.5 cm x 3 cm
Electrical requirements
     External cavity < 1 mW, 50 V < 1 mW, 50 V
     Laser diode driver < 100 mW, +/- 15 V < 100 mW, +/- 15 V
* Can be electronically placed anywhere within the total tuning range.
** Output power assumes a 10 mW laser diode.
*** Assumes the bare laser diode threshold current is 15 mA with uncoated facets.

Conclusions:

Vescent Photonics, Inc., designed a novel external cavity diode laser based on a giant EO effect, and experimentally demonstrated all of the basic material science and optical physics necessary to establish feasibility, including the new EO effect. A thorough design of the laser using the laser rate equations, which included the effects of material and coupling losses, was completed. In Phase II, Vescent Photonics, Inc., plans to build WECSLs at 670 nm, 1,824 nm, and 1,954 nm for spectroscopy-based sensing of NO2, NO, and N2O, the species comprising NOx.

WECSLs can be used for ultra-compact and robust trace gas sensors using tunable diode laser spectroscopy. The technology is compatible with laser diodes, including new 400-nm GaN lasers to IR lasers at 2.0 mm, so that a very large number of trace-gas species can be detected. These lasers are an alternative to DFB lasers, which typically temperature tune over 2 nm, have large linewidths, and generally are not available at any desired wavelength. In contrast, WECSLs are electronically tunable, have small linewidths, and can be made at any wavelength where Fabry-Perot laser diodes exist.

Supplemental Keywords:

external cavity diode laser, environmental monitoring, electro-optic effect, spectroscopy, trace gas, NOx, sensor, linewidth, WECSL, distributed feedback laser, small business, SBIR., RFA, Scientific Discipline, Air, INDUSTRY, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, Environmental Chemistry, Monitoring/Modeling, Analytical Chemistry, Environmental Monitoring, Atmospheric Sciences, Industrial Processes, Engineering, Engineering, Chemistry, & Physics, Environmental Engineering, trace element speciation, field portable systems, field portable monitoring, combustion gas streams, emissions monitoring, trace gases, emissions measurement, flue gas monitor, ambient emissions, air sampling, field monitoring, emission control, wavelength division multiplexing, combustion, diode laser

SBIR Phase II:

Robust, Tunable Diode Lasers for Environmental Monitoring  | Final Report