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. Contact Alpes Lasers SA in case of doubt or concerning limitations for a particular laser.
The first information source concerning the starter-kit is the corresponding manual. Please read it thoroughly before seeking additional information about the starter-kit.
QC lasers from Alpes Lasers SA are mounted on special carriers, which require special handling and definition of geometrical orientation.
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. The pads are yellow on top due to a layer of gold, and white around it and on the sides (colour of the ceramic). If these pads are placed upwards, the vertical for the laser is the same as the observer vertical direction.
If there are two ceramic pads present, they are named as follows: Looking onto the front facet with the laser placed as described above, the pad left of the laser chip is called "down", the one right of it "up". If no configuration is specified, the "down" pad is used.
There are two versions, the current and the new standard submount; dimensions for both are given in the drawings.
Fig.1 ST submount
Fig.2 NS submount
Never place the laser upside-down, since this will damage the bonds
connecting the pads to the laser!
Please refer to the image in the section Axes of lasers.
QCLs can be stored at ambient temperature (10..30degC) 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).
The laser should always lay flat (with its vertical axis upwards) on a flat and stable surfaces, without touching anything around its circumference. Of course, when mounted in an appropriate and stable holder, it can be operated in any orientation.
The most delicate parts of a QCL are the laser chip itself and the bonds connecting it to the ceramic pads. Therefore the QCL should be touched only at the copper carrier (far from the laser chip and the bonds), or at the ceramic pads (again away from the bonds).
To insert it into or to take it out of the starter-kit housing, gently grab the ceramic pad from above with fine tweezers, and whenever possible, carry the QCL placed flat on a stable surface. Take special care not to touch bonds nor the laser chip itself, since this can immediately destroy the QCL.
Avoid contact of the front facet of the QCL with any object (like the walls of a box where it is stored).
Depending on the type of the laser operation temperature (room or liquid nitrogen), different mounting techniques exist.
If you want to solder wires to the laser submount, please contact Alpes Lasers SA first, otherwise warranty may be lost! Here, the basic procedure and needs will be explained.
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.
It is of highest importance to never touch any part of the laser chip itself (especially not the facets) nor the bonds, as this may result in fatal damage to the QCL. Solder with a very fine tip solder iron at about 170C, only at the corner of the contact pad which is most distant from the bonds and the chip, and only very thin and flexible wires.
This chapter discusses electrical properties of pulsed and CW QC lasers; for special issues concerning CW operation, see CW mode.
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. Whenever you drive a laser at a duty cycle higher than specified, monitor the average output power; do not increase the duty cycle any more when the power saturates, but reduce it again to stay on the safe side.
If possible, increase the duty cycle by reducing the pulse period, not by increasing the pulse length, since the latter is more dangerous: It increases the short time heat load on the laser, instead of the average heat load.
Before doing such experiments, it is recommended to contact Alpes Lasers SA, otherwise the responsibility is with you.
You will see no decrease of the maximum instantaneous power of the device up to 2..5% depending on the device. Around 5..20%, the maximum average power will be obtained. Over this limit, the increase of average power due to increase of duty cycle will be smaller than its decrease due to increased threshold current (caused by higher average temperature of the structure).
The precise percentages depend both on the technology used (normal pulsed 2 mW or high power DFB) and the wavelength. For normal pulsed devices at short wavelength (4..5um), the maximum duty cycle is 3..5%, and at longer wavelength it may go up to 8% or even 20% for high power DFBs.
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.)
For information about thermal properties, See Heating and Cooling.
The impedance of a QCL depends strongly on the wavelength it is designed for, the temperature and the mode it is operated, therefore only rough indications can be given here (consult the datasheet of a particular laser for exact behaviour).
Pulsed mode devices have an impedance in the region of 5..50ohm up to about half the threshold current, then it decreases to the region of 0.5..5ohm. When operated at too high current, the impedance can rise again (a condition to be avoided in any case).
Certainly (as long as the applied current is not higher than 10mA), but it might not give you a lot of information, since the impedance varies strongly with the temperature of the QCL, and normal ohm-meters do not specify the applied current. Therefore, the measured impedance varies also with the resistance range of the ohm-meter, and between different ohm-meters. This is also the reason why Alpes Lasers SA does not specify the DC resistance of QCLs.
Generally, the cathode is connected to the ceramic pads and the anode is connected to the copper carrier. It may happen that the laser is mounted junction down; this case is clearly indicated on the laser box, and then the cathode is connected to the carrier and the anode to the bonding pads.
Unfortunately, it is in general not possible to use a standard laser driver for a QCL, as in most cases the compliance voltage, current and rise/fall time are not compatible.
Requirements for a pulsed QCL:
Alpes Lasers SA produces starter-kits which provide at the same time driving, temperature control and protection of the laser chip. See Starter kit. For a CW QCL, some standard laser drivers can provide the necessary conditions.
A CW QCL is about as sensitive to electrical surges and instabilities as a conventional bipolar laser diode (telecom NIR laser). It is necessary to use a good quality power supply to ensure:
We recommend precision laser diode current sources such as the ILX
Lightwave LDX-3232.
The emitted mode is single lateral, and also single longitudinal for the DFB devices. The divergence is 60deg FWHM in the vertical direction and 10 to 20deg FWHM in the horizontal direction (see the images on our website at http://www.alpeslasers.ch/technology/Technology.htm).
The QCL is based on a mechanism (inter sub-band transitions) that exhibits a poor efficiency: most of the electrons emit phonons instead of photons. The laser thus heats a lot and thermal management at the microscopic scale of the waveguide is important to allow operation. The waveguide is thus very small in the vertical direction (i.e. perpendicular to the quantum wells plane) in order to optimize the overlap between the optical mode and the gain region.
Depending on the wavelength and parameter optimised in a laser, it requires a different optimization of the width of the laser stripe. This results in a varying lateral confinement on the beam, thus various lasers at different wavelengths may exhibit pretty different lateral divergence. The lateral divergence is always smaller than the vertical 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.
The polarisation is vertical if the laser is laying horizontally, i.e the electrical field is perpendicular to the plane formed by the ceramic pads, see Axes of lasers. It is very pure as there is a quantum mechanical selection rule forbidding emission in the horizontal direction.
Due to the large divergence of the beam, it is recommended to use fast optics (f/1 ... f/0.8) to collect most of the emitted light. We recommend the lens system from Cascade Technologies (see http://www.cascade-technologies.com), and aspheres from Janos (see http://www.janostech.com) or Optical solutions (see http://www.opticalsolutionsinc.com).
The radiance can be estimated in two ways:
These are highly approximative values; if you need well defined ones, ask for the needed values for a specific laser you are interested in.
This chapter discusses properties of the Starter kit, used for pulsed mode lasers.
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...130W. Depending on duty cycle, the mean dissipated power normally is in the range of some Watts.
Normally, the TC-51 is shipped with current limitation of 5A and alarm value of 65C. Depending on the heat sink used, temperatures between -35 and +60C may be reached. Very high and low temperatures induce more stress on the Peltier elements and therefore accelerate ageing.
The pulse driver LDD100 can amplify pulses with lengths of 5ns...300ns and minimal period of 100ns. Maximal duty cycle is 50% (with reduced stability and not continuously to prevent overheating, up to 90%). The pulse generator TPG128 is capable of generating pulses with lengths of 20ns...200ns (with reduced stability down to 10ns) and period of 0.2us...10.5us. Duty cycle can vary in the range of 0.1%...80% (with reduced stability up to 95%).
In combination with LDD100, only 50% duty cycle can be reached, since the power supply in TPG128 which is feeding LDD100 is not specified for higher values. Provide external power source for LDD100 if more than 50% is needed. In any case, contact Alpes Lasers SA first, if duty cycles of more than 2% are needed.
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. Any standard laboratory power supply can be used, as long as it is ripple-free (<=1%), voltage regulated, with variable voltage from 0V to at least 35V, and capable of delivering 1A DC for duty cycles up to 2%. For higher duty cycles, contact Alpes Lasers SA, since not all lasers are capable of working at more than 2%.
This section describes use of a bias-T circuit for electrically controlled modulation of peak emission wavelength.
For modulation of a CW QCL, please refer to CW modulation.
The bias-T allows to apply a constant (DC) current to the laser in addition to the pulsed current (therefore a bias-T is useless in CW mode). The current is drawn from the external (user supplied) power supply through the laser.
This current can be controlled electrically. Alpes Lasers SA specifies use up to of 0.1kHz, but several clients have used the bias-T successfully at frequencies of up to several kHz.
Since tuning of a QC laser is done by changing the temperature of the active zone, the DC bias current can be used to control the emission wavelength of the laser via its heating effect. The bias-T therefore allows for electrically controlled rapid scanning of the emission wavelength.
Tuning can also be achieved by changing the temperature of the whole laser but at much lower speed, due to the high thermal capacity of the laser submount and laser base. Heating of the active zone alone by applying a DC bias current is affecting only the active zone and the surrounding parts of the laser chip, and due to the small thermal capacity of this tiny volume, the laser emission responds much faster to DC bias current variations.
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. Connections are different in the two cases:
Since the input stage of the bias-T is a bipolar transistor, applied voltage must be higher than about 0.6V to start bias current. The input stage has maximum voltage limit of 2.6V, but the laser itself may be destroyed at lower bias-T control voltage already, therefore the maximum rating has to be checked with the abovementioned rules and together with Alpes Lasers SA.
The monitor output (if available) allows measurement of applied DC bias current: Its voltage divided by 10ohm gives bias current. In general, bias current can be in the range of 0.1A, but this must be checked with Alpes Lasers SA before.
Avoid reverse polarity on the input!
For electrical properties, see Electrical 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!
To modulate a CW operated QCL (e.g, to sweep its emission), there are two possibilities.
If your DC supply allows for this, you should directly use its control input to change the output current (in the specifications given by the datasheet of course). Please make sure that its bandwith is sufficiently high for your application.
If your DC supply cannot be modulated (or not fast enough), you will have to add an additional modulated source, e.g, a waveform (ramp) generator. Alpes Lasers SA does not supply such generators nor the needed interfacing, but in the following, a scheme for setting up the interfacing is explained which has proven to work. Please refer to the drawings shown.
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. The values of R_AC,L,C depend on the modulation frequency, the AC source and (to some extent) on the QCL.
As the impedance X_L=2\pi f L of the inductivity should be much higher than the dynamic resistance X_Q of the laser to block the AC component from the DC supply, applying X_L>100X_Q leads to roughly L > 16 X_Q / f.
In general, X_Q is of the order of 1..2ohm. For a modulation frequency of 10kHz, L should therefore be of the order of 3mH or larger.
To prevent the AC source from dominating or reverse biasing the current through the QCL, U_AC < U_Q must always be given (in absolute values). Furthermore, as the AC voltage source together with R_AC,C is forming a current source, to prevent variations of QCL dynamic resistance X_Q to influence the AC current, R_AC+X_C >> X_Q must be guaranteed. As the generator for U_AC will have an impedance Z_AC, impedance matching demands for R_AC+X_C+X_Q=Z_AC. Putting these constraints together leads to I_AC < U_Q / Z_AC.
For the capacitor C to let pass the AC current, 2\pi R_AC C > 1 / f must be true, and with X_C = R_AC / 100 or smaller to make sure that the impedance of the AC current source does not vary too much with frequency, it follows that roughly C > 8 / Z_AC / f.
For a 50ohm 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.
To reduce cross-talk with other system components and to improve mechanical stability, this interface circuit should be installed into a small metal box with only the connectors accessible, as shown in the following figure.
CW QCLs are now commercially available for operation at cryogenic (LN2) temperatures (single- and multi-mode devices). Availability of CW FP devices for operation at room temperature is expected during 2004; currently no date for commercial availability of CW single-mode devices at room temperature can be given.
State of the art in research is that experimental multimode devices have been shown working, as well as DFB devices; however, better control of manufacturing is needed to make them available as commercial products.
This chapter discusses some general properties of QC lasers, mainly concerning optical behavour. For additional information, See Electro-optical
At Alpes Lasers SA, the following methods are used for detection and qualification of QC laser emission:
QC devices have been shown to be capable of operating between 3.44 and 84um. Devices with wavelength ranging from 4.2 to 11 um are actually available. Nevertheless we encourage you to discuss your needs if they lay outside this range.
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.
Using two different semiconductor materials (InGaAs and AlInAs), a series of potential wells and barriers for the electrons can be built. These wells and barriers are so thin that the electrons are allowed only a discrete set of energy levels. This situation is very similar to the orbitals of an electron around a nucleus in the case of an atom. In the case of the QC structure, the positions of the permitted energy levels are determined by the thicknesses of the wells and barriers.
It is thus possible, using only one material system (InGaAs/AlInAs grown on InP), to define laser transitions with energies ranging over a wide span. The limits are set, on the short wavelength range, by the potential difference between the wells and barriers, and on the other side by intrinsic absorptions of the material.
In conclusion, a QC laser is a laser made with some sort of a specific composit designed specifically for each wavelength but always composed of the very same materials.
For a slow detection system, a pulsed laser will appear CW. In order to define slow, See Line-width limit. A pulse will last between 10 and 100 ns and will be repeated at a rate corresponding to 0...3% for usual DFBs and up to 10...20% for high power DFBs. At 10 ns pulse length, the line-width will be close to minimal in pulsed operation, less than 0.1/cm, and at 100 ns it will be larger, depending on the device.
No standard output power ranges exist at this time, but Output power
in the range of 1or 2 mW can be expected for stock devices.
Normally, specification to 0.5/cm or 1nm should be sufficient. As the laser tunes over several linewidths, it is possible to temperature tune it to adjust its central wavelength. It becomes critical only if extreme power is required or if the laser has to operate CW at LN2 for the same reason.
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. This of course will result in an observable current tuning: the higher the current gets, the higher becomes the average temperature of the active region of the QCL. But this apparent current tuning is based on temperature tuning only.
The operation range of the device is -30...+30C (dT=60K). The relative tuning is constant for all wavelengths and is about 6E-5/K for wavelength and -6E-5/K for wavenumber. This results in a tuning range of about 0.4% of peak emission wavelength or wavenumber.
For a 1500/cm device, the total tuning is approximately given by -6E-5/K \times 60K \times 1500/cm i.e -5.4/cm.
Note: The relative tuning has a minus sign for wavenumbers and a positive sign for wavelength. This is exactly opposite to how a lead-salt device would tune, for those accustomed to this type of devices.
Like in every pulsed lasers, short enough pulses will lead to Fourier limited line width. For intermediate pulse length, the limiting factor is the thermal tuning of the device. The device heats up during the pulse and its emission wavelength follows and sweeps. Optimum pulses of 5 to 15 ns will enable to get a minimal linewidth.
Some customers such as the company Aerodyne published data on the linewidth reachable using our devices and electronics (starter-kit). Please have a look at http://www.alpeslasers.ch/Conference-Papers/Workshop-Freiburg-01.pdf which describes measured line width in pulsed operation.
The standard measurement setup we use enables to verify that the laser is single mode i.e. has a linewidth not exceeding 0.3/cm.
A FP-QCL tunes because of the shift in gain of the structure with temperature. This tuning is about twice as fast as the index tuning of DFB-QCLs, i.e about 1.3E-4/K (increasing temperature will result in increased wavelength).
On a Peltier cooler like the one included in the Alpes Lasers SA starter-kit, the obtainable temperature span is about 60K (-30...+30C). Therefore the central wavelength can be shifted by about 1.3E-4/K*60K or approximately 0.8% from the lowest to the highest temperature.
[1] In standard bipolar semiconductor lasers (e.g. 1.55um telecom devices), the emission wavelength is closely related to an intrinsic characteristic of the semiconductor material used, namely the band gap energy.