According to laser expert Sam Goldwasser, this laser contains the highest power 117A tube he has seen, outputting 180% more power than nominal and it has the vibration isolator built in! I also contacted several of the original engineers responsible for the design and marketing of the SP117A laser at Spectra Physics who confirmed it’s an excellent robust laser and intimated that it was discontinued only because it cost too much to produce.

I purchased this laser from Sam Goldwasser but it just won’t work for my application. Sam is a great resource and offered to answer any questions any buyer may have regarding this specific laser. E-mail him at sam@stdavids.picker.com if necessary.

BASIC SPECIFICATIONS:

Verified Output Power (measured before the isolator): 1.8mW

The Spectra-Physics Model 117A Helium-Neon Laser Class 3R (Illb) - Medium Power Laser

HELIUM-NEON LASING PROCESS

The helium-neon gas laser operates with an electrical discharge in a plasma tube containing a mixture of helium and neon gases. The discharge energy, through an intermediate helium meta-stable state, raises electrons of the neon atoms to an excited energy state. When more atoms are present in the excited state than in some lower state, a condition known as "population inversion" is said to exist. Optical radiation at the energy (wavelength) separation of these states can add to itself by stimulating emission from the excited neon atoms. In order to achieve continuous wave laser oscillation, a reflector is placed at each end of the plasma tube to form a cavity resonator which stores most of the optical radiation (photons) by reflection back and forth along the axis of the tube. The probability is about .1 that a photon will stimulate the emission of another photon in one pass through the tube. This means that the photons must reflect through the cavity a number of times in order to be self-maintained. Therefore only a small transmission is permitted through one of the reflectors; this provides the spatially coherent and monochromatic output beam characteristic of gas-laser operation.

When transitions to several different lower states are possible from the same excited state, the most likely transition will occur first and thereby reduce the population of the excited state; this makes oscillation with other transitions extremely difficult or impossible if the population difference is eliminated between the upper state and one of the lower states. This phenomenon is known as "dominance."

In attempting to obtain a particular output wavelength from the helium-neon laser, the dominance of transitions from the 3s2 orbital state of the neon atoms plays an important role. The most likely transition gives the 3391 nm infrared wavelength. To avoid oscillation at this wavelength, the laser mirrors have been designed to reject the 3391 nm transition; without this rejection, oscillations at 3391 nm would build up to the detriment of other desired oscillations in this unit. With the wavelength selection system, the operation is at the desired wavelength only; the transition of interest dominates all the other possible transitions from the 3s2 state to the several closely spaced 2p levels.

FREQUENCY STABILIZATION

The laser cavity in which the transitions occur is a resonant cavity. As is typical for such situations, only certain forms of oscillations can exist in a steady state: these are called the modes of the cavity. It is customary to separate discussion of the transverse modes from that of the longitudinal modes.

The transverse modes define the shape or phase of the wave front as it propagates in the cavity. Details of the mirrors and tube bore are used to confine virtually all of the radiation to the lowest order mode, TEM00. This mode gives rise to the Gaussian beam that is normally obtained.

In the longitudinal axis, a series of modes can exist. The permissible longitudinal modes correspond to those wavelengths for which one round trip in the cavity is an integral number of wavelengths, thus:

Nl = 2L

where N = the number of wavelengths, an integer

l= the wavelength of the radiation

L = the cavity length

For each value of N, a possible value of the wavelength is obtained which is a potential candidate for a steady state longitudinal mode. The separation of the modes in frequency is given by:

(delta) v = c/2L

where v = the frequency change

c = the speed of light

L = the cavity length

The laser transition which supplies energy for the gain is a very narrow line. This line is broadened by the Doppler shift caused by motion of the emitting atoms. The Doppler broadened line profile defines the nominal gain curve for the medium. For helium-neon lasers, the width of the gain curve is approximately 1300 MHz.

The number of modes which might be running in a laser is determined by dividing the width of the gain curve by the mode separation. For the Model 117A, there are two modes which operate in the cavity.

As the cavity length changes, as it will during warm up or as ambient temperature changes, the wavelength shifts due to the overriding requirement for an integer number of wavelengths in a round trip. This change results in the modes shifting along the gain curve to new positions with different amounts of gain.

The control circuitry in the Model 117A monitors the intensity of each of the two modes. In response to measured changes, a feedback signal is developed to control the tube length. This results in a very stable system in which the cavity length and tube temperature are accurately controlled. Beam rejection optics are employed to ensure that only one mode is emitted from the laser. The rejection of the 2nd mode is greater than 1000:1.

COMPLETE SPECIFICATIONS:

PHYSICAL

Power Supply Dimensions: 30.2x26.0x9.5 cm (11.88x10.25x3.72 in.)

Power Supply Weight: 3.3 kg (7.2 lbs)

Cylindrical Laser Head Dimensions: Length 40.1 an (15.8 In.), Diameter 4.5 cm (1.75 in.)

Cylindrical  Laser Head Wright: 1.0 kg (2.2 lbs)

ELECTRICAL

Voltage Required: 115/230 VAC 110%

Power Required: 40 W

Frequency; 47-63 Hz

ENVIRONMENT

Operating Temperature: 10 to 40°C (50 to 104°F)

Storage Temperature: -40 to 60°C (-40 to 140°F)

Altitude: Sea Level to 3000 m (10,000 ft)

FREQUENCY STABILIZED MODE

Frequency Stability:

        1 min 10.5 MHz

        1 hr 12.0 MHz

        1 day (8 hr period) 13.0 MHz

Frequency vs. Temperature: <0.5 MHz/°C

Temperature Range, Maintaining Lock: 20+/-10°C

Amplitude Stability: 1%

Modulation Feature (Nominal Value Only): 10 MHz/V

AMPLITUDE STABILIZED MODE

Amplitude Stability:   

        1 min <0.1%

        1 hr <0.1%

Frequency. Stability:

        1 min 13.0 MHz

        1 hr 15.0 MHz

Modulation Feature (Nominal Value Only): -2%/V

EITHER MODE

Output Power (product specification): IR 632.8 nm 1.0 mW

Output Power (measured before the isolator): 1.8 mW

Frequency (Nominal): 473.61254 THz

Beam Characteristics:

        Diameter 0.5 mm

        Divergence 1.6 mrad

Resonator Characteristics:

        Transverse Spatial Mode: TEM00                       

        Polarization: Linear >1000:1

Noise Power Spectrum:

        50/60 Hz: -100 dB

        Servo Loop: -90 dB @ 5 kHz

        High Voltage: -70 dB @ 24 kHz

Spectra-Physics, Inc.

1250 W Middlefield Rd

Mountain View, CA 94039

Tel: (415) 961-2550

Laser Products Division (800) 227-8054