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Engineering Constraints for
Continuous Tuning of External Cavity Diode Lasers (February, 2006) Justin T. Hawthorne, Amin R. Nehrir |
Abstract-Continuous tuning of
External Cavity Diode Lasers (ECDLs) has the potential to impact many aspects of
the world around us.� Throughout this
project, a primary goal has been to analyze these potential impacts in order to
fully understand the importance of applying sound engineering practices to the
entire design.� The following is a brief
summary covering the important aspects constraining the design choices made
throughout this project.
Index Terms� External Cavity Diode Laser (ECDL)
A-Political
The Federal Communications Commission (FCC) governs the maximum power output over given frequency ranges.� The spectral mask currently enforced by the FCC governs frequencies up to the 300 Gigahertz range.� The Proposed ECDL in this system will operate in the vicinity of 400 Terahertz, well above the cutoff frequency for the present FCC spectral mask.� Recent discussions have introduced the idea of broadening the present spectral mask to include frequencies in the terahertz range but no actions have been taken to date.� The Occupational Health and Safety Administration (OSHA) has provided guidelines for safe laser operation which will be discussed in the following section.
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B- Health
and Safety
Operating ECDLs require that stringent protective measures be taken to ensure sufficient safety standards are upheld.� The Occupational Safety and Health Administration enforced general industry standards such as (29 CFR 1910) which relates to every employees right to personal protection equipment.� The general duty clause, section 5(a)(1) of the OSHA Act, also applies.� Practical strategies which specifically apply to laser applications are given in Guidelines for Laser Safety and Hazard Assessment [1].� OSHA has also handled a handful of cases involving laser safety; however most of the topics have involved class 1 lasers (generally used in check out lines).� The laser that will be employed in the final product will be a class III b (output power ranges between 5 and 20 mW of optical power) laser with a maximum output power around 20 mW.� This means that the direct beam is hazardous to the eye and reflections of the direct beam can be harmful as well.� Precautions such as protective eyewear as well as proper warnings must be employed at all times in the vicinity of the laser.
C-Social
The potential applications of diode lasers with broad tuning ranges are mainly two-fold.� Wavelength Division Multiplexing employs tunable diode lasers due to the increased wavelength control.� The increased resolution of the output wavelength leads to higher data transfer rates over narrowed bandwidths.� Diode lasers are also used in spectroscopic applications to tune across absorption spectrums of various molecules.� The precise tuning capabilities of ECDLs allow for spectroscopic measurements with minimal error.� The environmental impacts of this are discussed further in the next section.���
D-Environmental
Molecular Spectroscopy is one of the most accurate tools available to analyze the molecular composition of the atmosphere.� The information gathered from this type of analysis is of particular importance to Ecologists and Meteorologists as it allows them to model the atmosphere based on actual molecular compositions of the different layers of the atmosphere.� Concentrations of greenhouse gases can be monitored with these processes; this is currently one of the largest markets for tunable diode lasers.��
E-Economic
Tunable laser diode systems are quickly becoming the most efficient way to get high data throughput rates with band limited channels.� If these trends continue, the laser diode market in telecommunications will reach 5.2 billion dollars by 2005 [3].� There is no evidence for these trends to fall off with 40 and 80 gigabyte per second throughput rates on the horizon.� Due to the precise noise thresholds of ECDL systems and the high cost of ECDL components, little attempt was made to minimize the cost of the individual circuit components.� The high cost of ECDL systems has somewhat relegated them to institutional use, but as the market grows, the price can be expected to fall. �
�
F-Ethical
The circuit for the current controller will be modified from the design implemented by Hall and Libbrecht in the literature, Low-Noise High-Speed Diode Laser Current Controller in order to achieve the performance desired by our system.�� The design process for the ECDL�s external cavity was aided by Dr .Kevin Repasky.� Should any part of the design become profitable (perhaps through manufacturing), the aforementioned contributors will be compensated justly.���
G-Manufacturability
Many elements required to characterize the final system are currently manufactured on the open market.� The optical detector, bias-Tee, laser diode and the lock-in amplifier will all be manufactured elsewhere.� The printed circuit boards involved in the fabrication of the servo and the current controller were manufactured by PCB Express.� The external cavity for the laser diode has been machined on the premises of MSU.� Integrating the complete system together will be the final manufacturing step.� Large scale manufacturability of every aspect of this project is plausible and much more cost effective than prototype production.� Certain circuit components were chosen to ensure sustainability
H-Sustainability
The circuit components used for the proposed designs (the current controller and the fast servo) are readily available.� �Two of the transistors used in the Libbrecht and Hall current controller design were no longer in production.� In order to replace the VP0106, a TP0606 was used due to its 60 Volt breakdown voltage and large drain current. �The 2n2905 was replaced with a similar switching transistor, the MPS2907A.� Standard component values are used throughout the circuit design to ease the replication or replacement of parts when necessary.� The laser diode itself is extremely fragile and susceptible to electrostatic discharge.� This is the main cause of laser diode failure and precautions such as anti-static bracelets (static straps) must be employed during the assembly and operation in order to ensure sustainable operation.� Documentation on the operating procedure must be sufficient to allow colleagues with similar technical backgrounds to operate the finished product and reproduce the design.��
I-Reliability
The robust design of the Littman Metcalf Configured ECDL will produce extremely reliable results.� When operated within the required design specifications, the current controller and the servo will need to be able to provide the bias current which drives the ECDL.� Comprehensive testing over the entire tuning range will be required to demonstrate the reliability of the final system.� Since one main advantage to the use of ECDLs in applications over rival technologies is the mobility of the complete system, ECDL system applications often require operational ranges that vary greatly from laboratory conditions.����
Tunable diode lasers are ideal for optical spectroscopy because of their narrow line widths, large tuning ranges and stable outputs. Because they are more compact and rugged than traditional spectroscopic optical sources, like dye lasers, and cathode lamps, they have enabled spectroscopic methods to be used not only in laboratory environments but also in the real world. Applications of diode-laser spectroscopy include remote sensing, LIDAR (Light Detection and Ranging), and trace gas detection. They can be used to monitor environmentally important things, such as methane, carbon dioxide, and water.
References
[1]
G. Schlenker,
�Guidelines For Laser Safety and Hazard Assessment,�
http://www.uwlax.edu/faculty/jackson/research/lasers/images/OSHA.htm,
[2]
W.-K. Chen, Linear
Networks and Systems (Book style).
[3]
Strategies Unlimited, Telecom Laser Diode
Market Review and Forecast.�� By
Global Information Incorporated, 2000.
�
Mr. Nehrir, from the beautiful town of
Mr. Hawthorne
was born in
�Manuscript received February 2nd,
2006.� This work was supported in part
by� PCB Express,
Todd Hawthorne is with the
Amin Nehrir is with the
��� Financial support for this project has been graciously donated by PCB Express and Dr. Kevin Repasky.
��� Intellectual support gratuitously donated by Dr. Gregg Switzer and Dr. Kevin Repasky.