01D. Items of Interest - General laser information

What makes a laser power meter so expensive?

Commercial laser power meters cost $300 and up - $1,000 is a more typical price for something that works over a wide range of power levels and wavelengths. Where the precision and automatic wavelength calibration of these instruments is not needed, a basic laser power meter can be built inexpensively. See the section: "Sam's super cheap and dirty laser power meter".

There are several ways to design a device that will determine the power in a beam of light. Here are two:

* Photodiode - each photon within the wavelength range of the device creates an electron-hole pair. When reverse biased, this results in a current flow which is proportional to light flux.

* Thermo-electric - the beam hits a sensor that absorbs (nearly) 100 pecent of the incident light energy at the range of wavelengths in question - a black body. This raises its temperature with respect to a known reference with a known thermal resistance between them.

Here are some comments on these approachs:

(From: Bill Sloman (sloman@sci.kun.nl)).

The important thing to note is that a photo-diode actually detects photons, not power. Up to about 850nm, each photon actually reaching the diode junction generates one pair of charge carriers. A 425nm photon, carrying twice the energy of an 850nm photon generates the same pair of charge carriers, so the same current represents the absorption of twice the power.

Since the 425 nm photon has rather less chance than the 850 nm photon of actually surviving the trip down to the diode junction, so the actual ratio is closer to 2.5:1.

Above 850 nm, the photons haven't got quite enough energy to separate a pair of charge carriers, and can only separate those that are already somewhat excited. The proportion that are sufficiently excited depends on temperature. A electric field also helps, so biasing the diode increases it sensitivity to long wavelength photons. As the wavelength rises above 850nm the extra energy required to separate the charge carriers also rises, so the proportion of 'sufficiently excited' carriers declines quite rapidly.

In principle one could build a wavelength correction into the power meter, but you would need to add a wavelength sensor to the power meter to make it a useful feature.

The Centronics data book gives a typical spectral response for the 5T series diodes, which effectively gives you the inverse of the wavelength correction function, albeit with rather low precision.

The alternative approach is to use a sensor which responds to the heating effect of the laser beam. These exist, but what you win on wavelength independent calibration, you lose on sensitivity and zero stability - in effect you have built a thermometer to measure the heating effect of your laser beam on a more or less thermally insulated target. Unless someone has done something very neat in this line, it doesn't strike me as a practical proposition for your application, granting your limited budget.

Sam's super cheap and dirty laser power meter

Hobbyists and experimenters may not need the super precision or automatic features of a commercial (and costly) laser power meter. For example, the wavelength or wavelength distribution of the laser source is almost always known. Therefore, if a correction needs to be computed using mushware (i.e., the stuff between your ears), so be it. There will be no absolute reference either but calibration using a source with known output power and wavelength like a 1 mW HeNe 632.8 nm laser will work just fine. And, if you really want a 16 digit LCD display, one can always be added :-).

I tossed this together using a 4 segment photodiode chip from a dead and abandoned Mouse Systems optical mouse (the old type which uses a pair of these chips - one for each axis). The active area of each segment is about 1 mm x 1.4 mm (total about 1 mm x 5.6 mm) which isn't great but is adequate to capture the entire beam of a typical collimated laser diode or HeNe laser.

A larger area photodiode would be better. To ease this a bit, I tied all 4 segments in parallel so one dimension is no problem at all. There are microscopic gaps between the segments but I estimate it to be less than 5 percent of the area so the loss should not be a big problem.

An 'instrument' (this term is being used very generously!) of this type will not replace a $1,000 commercial laser power meter but may be sufficient for many applications where relative power measurements are acceptable and/or where the user is willing to do a little more of the computation :-). One cannot complain about the cost: $0.00.

The basic circuit is as follows:

* The meter (M1) I used was a D'Arsenval moving coil type that had a full scale sensitivity of 10 mA. A suitable shunt can be used with a more sensitive meter or just use one of the current ranges of your VOM or DMM.

R1 provides current limiting to protect the meter movement from vaporization should the photodiode array short out. The combination of Vcc and R1 just needs to meet the requirement that the photodiode array remains reverse biased at the maximum expected current (optical power).

For the value of R1 shown above, Vcc should be at least 4 VDC for a photodiode current up to about 3 mA.

* I do not know the maximum ratings of this photodiode array but it seems to be fine with Vcc up to at least 12 VDC. Since current is nearly independent of the bias voltage, Vcc is not at all critical.

* Sensitivity is about .45 mA/mW at 632.8 nm from a HeNe laser. Though I have nothing precise to calibrate it against, the readings were consistent and linear with the tubes I tried which had their output power labeled.

* Mount the photodetector on a 'third hand' type of mount so it can be easily positioned in the beam path.

* A lens can be used to reduce the beam diameter of your laser so that the entire beam fits within the area of the photodetector. Where the beam profile exceeds the dimensions of the photodetector, an estimate of beam power can still be made knowing the ratio of sensor area to total beam area.

Unfortunately, with the small area of the photodetector, using this with intact CD laser optics may not be that easy.

* The range of wavelengths over which this is useful should extend throughout the visible spectrum into the near IR - at least until 850 nm or so.

I do not know what precise effect different wavelength lasers will have on the sensitivity of this circuit. Shorter wavelengths are more energetic but generate the same number of charge carriers (i.e., same current) and have less chance of surviving the trip through the diode junction. Thus, for a given photon flux the power reading will be low at shorter wavelengths. A correction factor can probably be computed.

* I also do not have any idea at what point the photodiode array will be damaged due to thermal effects. This is certainly not a problem for up to 10 mW as long as it is not focused to a sharp point.

A pair of op-amps can be added to provide more flexibility. The following circuit is substituted for the meter (M1), above. Any general purpose op-amps (e.g., 741) powered from +/- 12 VDC (for 10 V full scale) can be used.

This circuit provides 3 ranges. R7 (calibrate) allows the sensitivity to be adjusted for your particular photodiode and laser wavelength. With R7 set to 1.22 K, the ranges will be .01 mW, .1 mW, and 1 mW per V of Vout at 632.8 nm. Vout can also be monitored with a scope or connected to an audio amplifier to detect an amplitude modulated laser beam.

For the Range Select switch (S1), make-before-break contacts are recommended to prevent high amplitude glitches when changing ranges.

For my photodiode array, the dark current was insignificant. Should this not be the case with your device a potentiometer tied to a negative reference can be used to null it out by injecting an equal and opposite current at the (-) input to U2.

Many variations and enhancements to this circuit are possible.

About laser speckle and other phenomena

Speckle is a mottled pattern that arises when laser light falls on a non-specular reflecting surface. Lasers with high spatial and temporal coherence properties are likely to produce dramatic speckle effects. Thus, gas lasers like HeNe types are more likely to exhibit this effect than laser diodes.

For those applications where the laser's bright light and its ability to be sharply focused or easily collimated are important but coherence is irrelevant, speckle is an undesirable side effect to be avoided.

(From: Mike Poulton (tjpoulton@aol.com)).

If you want any more information on any kind of laser, or sources for parts to build them, post your question on alt.lasers. There, a group of about ten laser enthusiasts (including myself) will jump on your question and answer it in every possible way and in great detail.

As for the speckle pattern, that is usually called the interference pattern. It has nothing to do with your eyes and has no bearing on how well you can see as it is a real phenomenon. Laser light is completely monochromatic and is also in phase. When this light is scattered, it gets out of phase and the waves collide. When a wave at a low point and a wave at a high point collide, they cancel each other out (just like those noise-reduction machines that send out ambient sound 180 degrees out of phase, except this is with light). Where the light cancels itself out, there is a dark space, where it does not, there is a light space. This creates a three-dimensional lattice-work of light and dark spaces.

As you move around it, you see different parts of the lattice and your view appears to move. The more "saturated" the area is with light, the more impressive this effect is. I have a 15mW Helium-Neon laser, and its effect is incredible. To say that this is in your head is like saying that it is an optical illusion when you look at different sides of a house. One cool thing to try is shining the laser into flood light (while it is turned off). The reflective coating on the inside of the bulb makes this effect very intense.

(From: J. B. Mitchell (ugez574@alpha.qmw.ac.uk)).

Speckle noise arises because of the highly coherent nature of the laser light and can thus be reduced or eliminated by reducing the coherence of the source. One easy way of achieving this is by introducing a rotating ground-glass screen into the beam. Placing the ground glass at the focus of the beam reduces the temporal coherence by introducing random phase variations while maintaining the spatial coherence (ability for the beam to be focused to a point). Putting the ground glass in an unfocussed beam reduces both the temporal and spatial coherence.

Alternatively, if you need to maintain the coherence for your application (interferometry, for example) the you can reduce the size of the speckles by increasing the aperture of the imaging system.

(From: Steve McGrew (stevem@comtch.iea.com)).

I know of three ways:

1. Increase the spatial frequency of the speckle so that it is so high it ceases to be a problem.

2. Use only specular objects and sources.

3. Decrease the temporal and/or spatial coherence of your laser beam by running it through something like a rotating diffuser.

Difference between Fabry-Perot and DFB lasers

The Fabry Perot laser design is what most people think of: lasing medium with mirrors at each end.

(From: Dr. Mark W. Lund (lundm@acousb)).

A Fabry-Perot cavity is the standard run of the mill cavity with two highly reflecting mirrors bouncing the light back and forth, forming a standing wave. This cavity is not very frequency selective, theoretically you could have 1 mm wavelength light and .001 micron wavelength light in the same cavity, as long as the mirrors are the right distance apart to form a standing wave (and higher order modes make this easier than you might think).

A distributed feedback laser replaces the back mirror with a grating along the cavity axis. Instead of being reflected abruptly like a metal mirror would, the grating reflects a little over each part of the grating until at the back of the grating the light has petered out. Of course, since the light is being reflected by the grating the reflected light is always in the correct phase no matter if it was reflected from the front or back of the grating. The distributed nature of the reflection sharpens the cavity resonance and distributed feedback lasers are typically of much narrower bandwidth than the same laser with mirrors. Mostly seen in laser diodes, distributed feedback can also be done with non-linear optics, volume gratings, and other more esoteric optical elements.

(From: Bret Cannon (bd_cannon@pnl.gov)).

Fabry-Perot lasers are made with a gain region and a pair of mirrors on the facets, but the only wavelength selectivity is from the wavelength dependence of the gain and the requirement for an integral number of wavelengths in a cavity round trip.

DFB (Distributed Feed Back) lasers have the a periodic, spatially-modulated gain, which gives a strong selectivity for the wavelength that matches the period of the gain modulation. DFB lasers lase in the same single longitudinal mode from threshold up to the maximum operating power while Fabry-Perot lasers hop from one longitudinal mode to another as the current and/or temperature change. Most Fabry-Perot lasers lase on several longitudinal modes simultaneously though with some of these lasers you can find currents and temperatures where they lase on only a single mode.

There are also DBR (Distributed Bragg Reflector) lasers that have a Bragg reflector, that is a volume grating, as the reflector at one end of the cavity, which provides wavelength selective feedback. These lasers lase on a single longitudinal mode but the lasing hops from longitudinal mode to longitudinal mode to stay near the peak of the reflectivity of the Bragg reflector as temperature and current are changed.

Comments on various color lasers

(From: Mike Poulton (tjpoulton@aol.com)).

Laser diodes have only been able to produce red and infrared beams so far (at least commercially). There have been some research reports of green and possibly blue laser diodes but only operating in pulse mode, at reduced temperature, and/or with very limited lifetime. This will no doubt change as enormous incentives exist to develop shorter wavelength laser diodes for numerous applications.

The green lasers you see are either argon or frequency-doubled Nd:YAG (neodymium doped yittrium-aluminum-garnet). The argon laser is a very large and complex device, almost always putting out hundreds of times the power of your pointer. A Nd:YAG laser is usually even more powerful, but is often pulsed. Diode lasers are not used in laser light shows because they are never powerful enough. I am sitting here typing this while allowing my 15mW Helium-Neon laser to stabilize and warm up. Its wavelength is shorter, and it is 3 times more powerful than the pointer. When a red beam is needed in a laser light show, these are usually used because they are usually more powerful than diodes, and the beam is more visible per milliwatt because of it's shorter wavelength. Happy Lasing, and be sure to visit alt.lasers for any laser info you need!

Comments on spatial and temporal coherence

(From: Daniel Marks (dmarks@uiuc.edu)).

There are really two coherences associated with any source; spatial and temporal coherence. Probably the coherence you are referring to is temporal coherence, but both are important for holography.

The temporal coherence is related to the bandwidth of the source. The more narrow the bandwidth of the source, the longer the coherence length. HeNe lasers have a very narrow bandwidth, as a result they have a coherence length on the order of 10-30 cm. LED's are incoherent sources, they only have a coherence length of 10-40 microns, and a large bandwidth of several kT (25.9 meV at 298K) or I'm guessing 10 nm of bandwidth (around about 650 nm). HeNe lasers are also much more spatially coherent than LEDs. The spatial coherence length is determined by the cavity and cavity reflectivity in a laser. LEDs also have a very short spatial coherence length, or only a couple of wavelengths.

The coherence length is the maximum distance at which two points in the field can be interfered with contrast. The temporal coherence length determines the maximum depth of the object in a reflection hologram, and the spatial coherence length determines the lateral size. Using techniques of "white light" interferometry, incoherence sources can be used, but they are tricky and have many restrictions on the kinds of holograms one can create.

Laser beam collimation

(From: Kai-Martin Knaak (kmk@physik.uni-mainz.de)).

There is a maximum distance that a beam of light can be kept collimated. Usually it is called 'Rayleigh length' and it depends on the wavelength and the minimum diameter of the beam. If the beam diameter is w0 at point z, then the beam will have expanded to at least 1.4 times w0 at Rayleigh-length distance from z.

The Rayleigh length, z_rayleigh, can be calculated like this:

_rayleigh = pi * (minimum diameter)/(wavelength)

For example, assuming a HeNe laser (632.8 nm) and a minimum diameter of 6 mm this makes about 180 meters. In practice, you might not get that far but 50 meters may be feasible. (Reality enters due to the fact, that the axial intensity distribution is assumed to be perfectly gaussian.)

One way of doing the collimation, is with a telescope consisting of two identical plano-convex lenses. If the lenses are spaced at the double focal length, their effect onto beam divergence will vanish. Putting them nearer increases divergence and moving them farther apart focuses the beam. So you can collimate the beam by fine tuning the lens distance.

Of course lens aberrations limit the performance, so weak lenses or aspheric lenses might be desirable. Spherical aberration will be reduced by turning the curved sides of the lenses face to face.

See the book "Lasers" by A. E. Siegmann for the details of the propagation of laser light. (page 664 ff.)

General comments on lasers as a hobby

We have addressed the issues involved in using common laser diodes and HeNe laser tubes. If you are really serious and want to go further, here are some comments on a variety of lasers.

(From: Richard Alexander (RAlexan290@gnn.com)).

How much do you like to build things? Would you prefer to assemble a bunch of parts, or do you want to blow your own glass tubes, too? Do you have any mechanical experience? Do you build electronic kits? Keep in mind that you will often be working with intense light (enough to instantly damage your unprotected eyes, and maybe your unprotected skin) and high voltages.

All laser experimenters (and optics types, too) should have a copy of "Scientific American"'s "Light and Its Uses." It gives construction plans for a Helium Neon (you blow the glass tube yourself), an Argon Ion (even more complicated), a CO2 (designed and built by a high school student, and able to cut through metal), a dye, a nitrogen (a great first laser, but watch out for UV light) and a diode laser (obviously, you buy the diode laser and assemble the driver circuit from the plans they supply). They also explain how to make holograms using visible and infrared light, microwaves and sound. There are other projects, too. The book is getting fairly old (the HeNe dates to the '60s or '70s), but it's still a great reference.

A nitrogen laser may be built for under $200 (maybe less than half that amount if you are lucky). It requires no mirror alignment (since it has no mirrors). The technology for building this laser was available to Ben Franklin, so there is nothing too critical in it. The hazards it presents are lots of ultraviolet light (spark discharges and laser beam), high voltage (necessary to arc across a 1/4 inch spark gap in a nitrogen environment) and circuit etcher (the main capacitor is made from an etch circuit board).

Once built, the nitrogen laser can drive many other projects. It can be used as a pump for the dye laser, for example. It will light up anything fluorescent. It is a pulse laser (10 ns) that can be repetitively pulsed (120 Hz is a likely frequency). Megawatt power is possible, but the total energy is low (due to the short pulses).

"Electronics Now" (formerly, Radio Electronics) has a laser projects column that started several months ago. I'm trying to think up a project I can submit to them. They said they would welcome projects for the laser column.

Helium Neon laser tubes may be bought from many mail-order companies. I bought one from Meredith Instruments in Arizona. They cost about $15, and the power supply can be built or bought for about another $20. You have the option of buying tubes with mirrors attached or not. You might want to buy the mirrors attached, because aligning those mirrors is extremely tedious. I was given an "A" for constructing a working Helium Neon laser from the parts in the Laser Lab in less than an hour. The class was given two semesters to gain the experience they needed to do that.

If you want more than one color from lasers, there are various ways to do it, but none of them are as nice as one might like. For $3000 or so, you can buy a Helium Neon laser that will produce laser light ranging from infrared to blue. All you have to do is turn a dial on the back.

Laser light shows usually use Argon Ion or Krypton lasers. These are able to produce most of the colors of visible light, and they can also be dialed to the desired color. However, they usually cost several thousand dollars ($40,000 is not too unusual) and require either forced air or water cooling or a combination.