QCL FAQ List Frequently Asked Questions about Quantum Cascade Lasers and Starter Kit from Alpes Lasers SA This FAQ should address the main questions arising for and from operation of CW and pulsed mode QC lasers from Alpes Lasers SA, especially in combination with the Starter Kit. The information given herein is based on best knowledge, but since lasers can behave differently, no guarantee can be given that it will hold true in any case.
Contents 1 Mechanical and geometrical properties 1.1 Geometry of QC lasers . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 How are the axes of the laser defined, i.e. what is vertical? 1.1.2 What are the physical dimensions of laser carriers? . . . . 1.1.3 What materials are used for the laser carriers? . . . . . . . 1.2 How to handle a QCL . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 How do I store a QCL? . . . . . . . . . . . . . . . . . . . . 1.2.2 How do I handle (carry) a QCL? . . . .
CONTENTS 2 3.5.2 What is the reflectivity of the LLH window? . . . . . . . . . . . . . . . . . . . . . . 4 Operating QCLs in continuous wave mode 4.1 Thermal properties . . . . . . . . . . . . . 4.2 Modulation of a CW mode operated QCL . 4.2.1 Direct modulation of the DC source 4.2.2 Additional modulated supply . . . . 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 1 Mechanical and geometrical properties QC lasers from Alpes Lasers SA are mounted on special carriers (NS or ST), which require special handling and definition of geometrical orientation. 1.1 Geometry of QC lasers 1.1.1 How are the axes of the laser defined, i.e. what is vertical? The vertical direction is the so called growth direction. In practice, you have a device in front of you, it is mounted on a copper carrier. The carrier has one or two ceramic pads carrying the bonding wires.
CHAPTER 1. MECHANICAL AND GEOMETRICAL PROPERTIES 4 1.1.2 What are the physical dimensions of laser carriers? There are two versions, the ST and the NS; dimensions for both are given in the drawings. Never place the laser upside-down, since this will damage the bond wires connecting the pads to the laser! 1.1.3 What materials are used for the laser carriers? The carriers are composed of a OFHC gilded base, one or two ceramic gilded connecting pads and AIN gold plated bonding wires.
CHAPTER 1. MECHANICAL AND GEOMETRICAL PROPERTIES 5 1.2 How to handle a QCL 1.2.1 How do I store a QCL? QCLs can be stored at ambient temperature (10..30C) in normal atmosphere. Humidity should not excess about 80%, and condensation is to be avoided. When operated, only dry atmosphere (below 50% relative humidity) is allowed, and if possible, it should be completely dried (desiccant material, N2 atmosphere).
CHAPTER 1. MECHANICAL AND GEOMETRICAL PROPERTIES 6 As the QCL chip itself is soldered, and the contact pads on the ceramics are made of Gold, it is not possible to use normal Lead-Tin solder (temperature too high, and solder will destroy Gold contacts by forming alloy). Alpes Lasers SA recommends use of pure Indium for soldering, in the form of paste of microscopic beads in flux.
Chapter 2 Electrical and optical properties This chapter discusses electrical properties of pulsed and CW QC lasers for special issues concerning CW operation, see Chapter 4, (CW mode), p.16. 2.1 Electrical limits 2.1.1 What is the maximum allowed duty cycle? This strongly depends on the laser. As a general rule, most lasers sold by Alpes Lasers SA are capable of being driven up to 10% duty cycle with pulse lengths up to 100ns.
CHAPTER 2. ELECTRICAL AND OPTICAL PROPERTIES 8 2.1.3 What is the lifetime of the laser (MTBF)? At present only extrapolated lifetime experiments have been performed and they show more than 10 years extrapolated lifetime at 20C. The measurements have been done operating devices under N2 atmosphere at 130C in pulsed mode at 130% of threshold current. (An activation energy of 0.7 eV has been used to convert the high temperature life time of 350 to 500 hours to room temperature life time.) 2.
CHAPTER 2. ELECTRICAL AND OPTICAL PROPERTIES 9 • maximum rise/fall time of 10ns (to prevent detrimental heating) Alpes Lasers SA produces Starter Kits which provide at the same time driving, temperature control and protection of the laser chip. For a CW QCL, some standard laser drivers can provide the necessary conditions. 2.2.5 How do I drive a CW QCL? A CW QCL is about as sensitive to electrical surges and instabilities as a conventional bipolar laser diode (telecom NIR laser).
CHAPTER 2. ELECTRICAL AND OPTICAL PROPERTIES 10 Is it possible to reduce the divergence? The divergence in the vertical direction is a parameter that is governed by the thickness of the laser waveguide. It is high because the waveguide is narrow. Reducing the divergence would impair the performances of the laser. Moreover this modification would need tremendous development effort and it would be necessary to compromise on the power and operation temperature. 2.3.
Chapter 3 Starter Kit (pulser, temperature controller, etc...) This chapter discusses properties of the Starter Kit, used for pulsed mode lasers. 3.1 Operation of TE cooler 3.1.1 What is the dissipated heat of a pulsed QC laser? Pulsed QC lasers in general work at threshold voltages of 9V...12V and threshold currents of 1A...3A, with maximum values of up to 25V and 10A. The peak power during operation therefore can vary in the range of about 10W...1000W.
CHAPTER 3. STARTER KIT (PULSER, TEMPERATURE CONTROLLER, ETC...) 3.2.2 12 What is the ”external power supply” used for? The ”external power supply” is a DC power supply provided by the user; it is connected (via banana plugs) to the pulse driver LDD100 and is delivering the electrical power feeding the laser.
CHAPTER 3. STARTER KIT (PULSER, TEMPERATURE CONTROLLER, ETC...) 13 thermal capacity of this tiny volume, the laser emission responds much faster to DC bias current variations. 3.4.2 What are the connections of the bias-T circuit? The bias-T circuit is either separately attached to the low-impedance line connecting the pulser and the laser housing, or directly included in the pulser.
CHAPTER 3. STARTER KIT (PULSER, TEMPERATURE CONTROLLER, ETC...) 14 the pulse current must always result in increased optical power output, otherwise the DC bias current is already too high. • As a rule of thumb, the overall dissipated power (sum of DC bias current dissipation and pulse current dissipation) must never be higher than the average dissipated power given by the highest current / voltage / temperature combination specified in the datasheet.
Chapter 4 Operating QCLs in continuous wave mode For electrical properties, see ”Chapter 2, p 8, Electrical properties”. 4.1 Thermal properties The dissipated heat of a QC laser operated in CW mode is in the range of some Watts (operating voltage in the 8V...12V range, current in the 0.5A...1.5A range). Keep in mind that in general, the impedance of a QCL is decreasing with temperature! 4.2 Modulation of a CW mode operated QCL To modulate a CW operated QCL (e.
CHAPTER 4. OPERATING QCLS IN CONTINUOUS WAVE MODE 16 Connectors for the LLH with a BNC plug are available for CW laser operation. This is always center positive, shield negative, and can directly be connected to a stabilized DC supply; make sure polarity is correct and limits as shown in the datasheet are set. To modulate the QCL, an AC signal needs to be added to the DC current. To prevent current modulation to go back to the DC source, an RLC circuit needs to be added.
CHAPTER 4. OPERATING QCLS IN CONTINUOUS WAVE MODE 17 does not vary too much with frequency, it follows that roughly C > 8/ZAC /f . For a 50 Ω generator at 10kHz, C should therefore be of the order of 16uF or larger. Please keep in mind that such high capacity components may have a remarkably high inductivity, so it may be useful to put a second capacitor of small value (some nF, Tantalum or similar non-polarized type) in parallel to the one calculated before.
Chapter 5 General QCL questions This chapter discusses some general properties of QC lasers, mainly concerning optical behavour. For additional information, see ”Electro-optical”. 5.1 How to measure QC laser emission? At Alpes Lasers SA, the following methods are used for detection and qualification of QC laser emission: Power meter To measure power, semiconductor power meters are used, in particular the combination of power head 2A-SH with meter AN/2 from OPHIR(www.ophir.com).
CHAPTER 5. GENERAL QCL QUESTIONS 5.2.2 19 Why is such a large range obtainable? This peculiar characteristic is due to the fact that in QC devices the emission wavelength is determined by the geometry of the semiconductor layers that compose the laser crystal.1 More precisely, the laser transition is the transition of an electron inside sub-bands from one upper quantum well level to a lower quantum well level. For more details we would encourage the reader to consult semiconductor physics text book.
CHAPTER 5. GENERAL QCL QUESTIONS 5.3 5.3.1 20 Tuning and linewidth How does a DFB-QCL tune? In QCLs there are no effects such as carrier density dependent index of refraction, therefore no current tuning is observed. The only tuning mechanism is temperature tuning of the index of refraction of the waveguide that changes the apparent optical length of the wavelength selection grating.
CHAPTER 5. GENERAL QCL QUESTIONS 21 can be shifted by about 1.3E-4/K*60K or approximately 0.8% from the lowest to the highest temperature.
Chapter 6 Glossary and Abbreviations CW Continuous Wave; for lasers this means operation with DC current, generating uninterrupted emission. DFB Distributed Feed-Back; describing a laser with an etched grating close to its active zone, which acts as a filter, reducing overall gain for all but the wavelengths defined by the grating period. This technique allows to produce single-mode lasers also for pulsed mode operation.
