Types of Lasers
There are many kinds of laser that have been used to make holograms. Below is a list of the most common types.
HeNe lasers were the most common laser used in amateur holography until the advent of the diode lasers. The are still often used because they have are inexpensive, have reasonable coherence length and the color is highly suited to available recording materials.
A HeNe laser is a glass tube filled with Helium and Neon at about 5 Torr of pressure. There are electrodes placed inside the glass tube and a high voltage is passed between the electrodes. This causes the gas to glow like a neon sign.
A mirror is placed at each end. They are usually reflective at 633nm (orange). One mirror is almost 100% reflective and makes the back of the cavity. It is called the HR (Highly Reflective) mirror. The other mirror makes the output side of the cavity and is called the OC (Output Coupler).
Some HeNe lasers have mirrors that are external to the glass tube and some have mirrors bonded into the glass tube. With external mirrors, the ends of the tube can be fashioned to the Brewster's Angle and then the laser will have a Polarized output.
HeNe lasers are commonly available from .5mw to 50mw in output power. They are often TEM00. They can be polarized 100 to 1 and have a coherence length of a few inches without an etalon. Some large HeNe lasers are designed to accept an etalon and can have coherence lengths measured in meters.
While a HeNe laser has about 20,000 hours of expected lifetime there are many modes of failure:
- As electrodes age they start to give of metal that gets deposited inside the laser tube. When this contaminates the mirrors or the ends of the tube in an external mirror the laser starts to drop in power. This deposition can often be seen as a black deposit inside the tube and signifies the aging of the laser.
- Seals inside the tube can start to leak and allow air to leak in.
- The high voltage power supply can also fail.
The power supply for a large HeNe can deliver 8ma at 6,000 volts! When servicing a HeNe laser it is very imortant that you understand the safety of the high voltage power supply. See Laser Safety for more information.
Diode Laser are the fastest growing market of lasers. They have become inexpensive because they are used id CD/DVD reading and writing technologies. Red laser pointers contain a laser diode and most can be used for holography (as can most <5mw red diodes).
This image shows the small diode chip resting on the housing with a blob of solder where the wire was attached.
A diode laser is an interesting beast for holography. It has long coherence length but the frequency dependence on temperature is extremely critical.
From Tom B.
As a first approximation, coherence length = (wavelength ^ 2) / (2 * linewidth) e.g. for 670 nm center wavelength and line width (wavelength range) of 0.2 nm, coherence length = (670 * 10E-9)^2 / 2*(0.2E-9) = 0.0011 meter Exact value would depend on the shape of the line.
The frequency equivalent:coherence length = speed of light / bandwidth e.g. for 1500 MHz, this is 3E8 / 1500E6 = 0.2 meter. (Iovine's book had a typo in this equation, but his example was correct)
Math and example from Iovine, "Homemade Holograms", 1990.
The rule of thumb for the temperature dependence of a visible single mode diode is .3nm/degree C. (Recently it was pointed out to me that visible laser diodes can have a slope of .18nm/C.) Fortunately this is not a continuous function. Otherwise we would need to hold a diode stable to within .00003C for a 20M coherence length! In-between mode hops the slope is much flatter. But even if it is .01nm/C in-between mode hops that works to a needed stability of .001C for a 20M coherence length. From my experience I can propose a rough guess of .05nm/C for inbetween mode hops.
Soldering to a diode takes some practice if your are not an experienced electric technition. See Soldering to Laser Diodes for some tips
The output of a diode laser is usually elliptical and you can use optics to circularize them. See the article Circularize an Elliptical Laser Beam.
For useful background info on operating diode lasers, see the ILX application notes.
Stabilizing a High Power Laser Diode
It is possible to stabilize the temperature of a laser diode with relatively simple setups. The necessary components are: A collimator that fits into a aluminum housing with at least one flat surface, a Peltier element, a cooling fin, a constant current power supply and a PI (Proportional Integrating) temperature controller. All parts are quite inexpensive and can be bought through the Internet. One such setup is as follows:
- Aluminum block with hole drilled to tightly contain the laser colimator housing+lens.
- Laser collimator housing + lens + laser diode.
- Aluminum base plate, several mm thick. If you make the base plate slightly larger, as is depicted here, enough space is left in front of the laser for other cool optical components, such as polarizers or an ECDL setup. Determine the amount of extra free space by turning on the diode with the collimator lens removed. The base plate should not block any light of the laser. The setup works great for Denisyuks when the lens is removed.
- In a little hole that is drilled, a NTC must be embedded in the base plate. Because, for some reason known only by the gods of microelectronics, NTC resistors have no flat sides, the NTC resistor needs to be fixed tightly against the Peltier element on the other side of the hole with a thermally conducting epoxy.
- The peltier element. Don't be tempted to try to control the temperature with a heating element instead of a Peltier. The PI controller will not be able to keep the temperature as constant with heating only.
- A cooling fin to remove heat that is transferred by the Peltier element from the base plate when in cooling mode.
Now for the power supply (from SAM's Laser FAQ):
The only adaptation from the schematic shown in Sam's Laser FAQ is the 1 Ohm series resistor. The measured voltage over this resistor equals the current that flows through it (beats the heck out of removing the diode connectors and measuring the actual current with a multimeter in series with the diode). The LM317 was originally designed for constant Voltage power supplies, but with the little trick in this circuit it works as a very stable constant current source for less than a few dollars. The LM317 has been designed to keep the Volage at a constant 1.25V between the middle and right terminal. Because the two terminals are connected with the two resistors in-between, it is the value of these two resistors (V=I*R) that determine the current that comes out of the right-hand terminal. Only a few nA will flow back into the middle terminal, the rest goes into the diode. The power supply to this constant current source needs to be stable and a few volts above the Voltage that is consumed by the laser diode. The capacitors can be cheap metal film capacitors, but preferrably not electrolytic caps.
How to hook it all up together:
It is important to note that all components need to make good thermal contact in this setup. I have used thermal conducting epoxy for gluing all components together. When using a good PI controller (such as the HTC1500 or HTC3000 + evaluation board), it is possible to keep the temperature of this setup within 0.001C. Holograms look deep and sharp, and no visible mode hops when a diode current or temperature is chosen where the diode output is stable.
Here is how I use the laser when it is in "Denisyuk mode" (collimator lens removed). It works every time with about 1 minute of warmup time and have not seen a single mode hop since. To the right the temperature PI controller can be seen. This is a HTC3000 with evaluation board. It needs to be powered with a power supply that can supply at least 4A, at about 9V. I do advise to use an evaluation board when you purchase a PI controller because they come with all the switches and trimmer pots to make the system almost plug and play.
Sources for parts:
50mW HL6512MG laser diode (tested in this setup): http://www.thorlabs.com
Diode Housing + Collimating Optics: http://www.mi-lasers.com/cgi-bin/shopper.cgi?search=action&keywords=diode_optics
TEC Controller + Evaluation Board: http://www.teamwavelength.com/products/product.asp?part=6
DPSS lasers use a 808nm laser diode to pump a 1064nm Nd:YAG laser, which uses an intra-cavity KTP crystal to double the frequency to 532nm. Nd:YVO4 can also be used as the gain medium. It can be frequency doubled with BBO to 457nm or 532nm.
Generic DPSS lasers, such as green pointers or cheap constructions, will in general not be useful for holography because they won't be single frequency lasers, which means they won't have a decent coherence length. Moreover, without active temperature control they won't be stable enough (exceptions may apply). As a rule, only DPSS lasers specifically built for single frequency operation are suitable. A commonly used example is the Compass 315M-100 laser with 100mW output, which, as well as its higher powered cousins, has a proven track record to be excellently suited for holography purposes. It is relatively easy to obtain from the surplus market, and typically goes for anywhere from $250 for a bare laser head to $1500 for a complete system including power supply.
A common method for making a single frequency DPSS laser is a ring laser. A ring laser has a traveling wave where all of the light only goes in one direction. The single traveling wave is obtained by inserting a Faraday Rotator and a polarizer into the cavity. For more information see Ring Laser.
- DPSS Lasers from WikiPedia
- Koechner, Walter (1992). Solid-State Laser Engineering, 3rd ed., Springer-Verlag. ISBN 0-387-53756-2
- A great description of DPSS ring lasers can be found in Christoph Boling's Thesis.
Argon Ion Lasers
These are the high-powered workhorses of many professional holographers. They can emit several wavelengths, the primary ones are 514nm (green), 488nm (cyan) and 476nm (blue). The power ranges typically from 10-20mW for small air-cooled types, to hundreds of mW and several Watts for large frame lasers. The power consumption tends to be enormous, even the smallest ones require approx 1kW to run, and the larger ones much more so they need to be water cooled.
By far not all argon lasers are suitable for holography, and especially the air-cooled types easily available on the surplus market are problematic, because they often do not fulful some basic requirements as they are made for other purposes (eg as components of printing machines). The three basic requirements for an argon laser to be suitable for holography are:
- single line operation: this means operation on only one of the lines 514nm, 488nm, 476nm, etc. This can be achieved either by using specific mirrors ("single line optics"), or by using an intracavity "Littrow" prism. The latter allows to quickly change the wavelength by simply tilting the prism. Small air cooled argon lasers, like the ones of Cyonics/JDS, Lasos, NEC, most often have internal mirror tubes, and so do not allow any changes. Other common types like the ALC 60 have external mirrors, but often have multi-line mirrors installed. Littrow prisms are mostly a feature of larger water cooled ion lasers made for research, eg from Coherent, Lexel, and Spectra Physics (now Newport).
- TEM00 mode operation: this is not always granted. For example, Lexel 88 lasers made for ophthalmic use do not have pure TEM00 mirrors installed by default; similar for ALC 60.
- Single frequency (single longitudinal mode) operation: the gain bandwidth of an argon laser is typically in the order of 10Ghz, which means that generically many modes will lase simulaneously, unless prevented from doing so. 10Ghz corresponds to a length of a little more than an inch (3cm), so this translates to a coherence length of this order of magnitude. This means that the maximum recordable depth of a hologram will be only an inch or so. In order to prevent more than one mode to lase and thus to ensure single frequency operation, the method of choice is to use an intracavity etalon. This cannot be done, of course, for most common air cooled lasers with internal mirrors. Retrofitting a laser with external mirrors with an etalon requires a massive reconfiguration of the resonator, and works well only for an etalon that has been optimized for that laser. In short, it is by far the best option to acquire an argon laser that comes already with an built-in etalon, anything else gives a lot of problems and most likely leads to unstable operation with low power.
Air cooled argon lasers are also problematic due to excessive vibrations from the fan(s), and air currents and temperature gradients from the 1KW or more of dissipated heat. Taking everything together, air cooled ion lasers are not well suited for holography purposes, and given that their power is typically <100mW, a DPSS laser is definitely a better choice. On the other hand, water cooled ion lasers equipped with etalon and single line/single mode optics are perfectly suitable, and surplus ones tend to get cheaper all the time due to market competition with DPSS lasers; used small and mid frame used lasers typically go between $1000-$2500 including power supply.
This is the Spectra Physics 165 Argon Ion water cooled three phase laser with the cover on.
This is the power supply for the Spectra Physic 165 Argon Ion water cooled three phase laser.
This is the rear of the laser. At the far left you can see the verical and horizontal adjustments. The horizontal changes wavelengths but the verical needs to be adjusted slightly when changing wavelengths. I usually move the vetical slightly toward the wall (back for blue) until the beam is dim to adjust the horizontal also back for blue. The opposite for green. Just to the right of the tuning belts you can see the air spaced etalon. It also has a vertical and horizontal adjustment. I adjust till peaked at flash point then move just the horizontal until the fringes are stable and contrasty.
This is the front. It has an adjustable aperture (dial with numbers on it) and a beam splitter to steal some light for the built in power meter on the power supply. It is barely noticable but on the right under the black square. The solenoid at the far left with the very fine tube is the refill solenoid which is used via the power supply to add addition argon gas when the tube voltage drops because of low argon gas. I usually have to do this once a year with my minimal usage.
This is the entire laser looking from the rear.
Helium Cadmium Lasers
HeCd lasers operate at 441.6nm. They can be TEM00 with coherence lengths of around a few cm.
Other CW Lasers
Krypton ion lasers are among the highest powered CW lasers in the yellow-red color range. For most aspects except wavelength, they are very similar to Argon ion lasers. They are rarely available as surplus.
A pulsed laser is one of the most exciting lasers to have for holography. It lowers the stability requirement by shortening the exposure time to the range of 20ns.
The two lasers most often used are the Pulsed Ruby Laser and the Frequency doubled Nd:YAG or Nd:Glass lasers. These lasers have been very expensive. However, lately there has been work done converting the SSY-1 Nd:YAG laser to work for holography. This may make pulsed holograms for around $1500 in cost.
Frequency Doubled Pulsed Lasers work in the 532nm wavelength of green and often require post-swelling of the plate to make the hologram color more golden but special makeup not required.
Pulsed lasers for holography are usually a MOPA design in order to obtain enough power.
RotorWave had great notes on designing and building pulsed lasers. The Holography Forum has mirrored some of it's files. A more updated version may be available on Laser Sam's FAQ.
Here is a page of Adam's SSY. This is a ruby pulsed laser frequency doubled for about $400! It is capable of small hollograms.