7 WAVEMETER

Measure the Wavelength of An Unknown laser Using 633nm and 543 nm HeNe lasers

MODEL OEK-100

PROJECT #6

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7.1 Introduction A wavemeter can be constructed with a Twyman-Green interferometer. The principle involved is that for the same geometrical setup of an interferometer, the number of fringes observed is dependent on the wavelength of the light. This fact can be utilized to determine the wavelength of a coherent source by comparing its interference pattern to the pattern of a known wavelength .

Two techniques are used in this experiment:

1. Measuring the frequency , fl ' of moving fringes of one wavelength, AI, and comparing to the frequency, 6, of the known wavelength, A2.

f} Al = f2 A2

2 . Counting the number of fringes, N I, that pass by for one wavelength, AI , for a given distance and comparing with the number of fringes , N2, of another wavelength, A2.

N t Al = N2 A2 There are many applications where it is necessary to precisely determine the wavelength of a source. This is particularly true with lasers, and the coherence properties of lasers enable the use of interferometric techniques to be utilized to determine their wavelengths. Accuracies of one part in 107

have been achieved but with much greater difficulty . In this experiment, we will concentrate on interferometric methods applicable for CW coherent sources.

The interferometer of choice for this experiment is a Twyman-Green interferometer and the concept involves counting fringes . As one arm of the interferometer is scanned, fringes are scanned through given by the equation rnA =2nd where m is the number of fringes scanned through, d is the distance that the Twyman-Green interferometer mirrors is moved and n is the index of refraction of the transmission medium (typically air n= l) . As can be seen from this equation, the number of fringes observed are dependent upon the wavelength for a given mirror displacement. This fact can be utilized to determine the wavelength of a coherent unknown source if one compares the interference pattern of the unknown source with a known wavelength source, i.e . for a given displacement one will detect a different number of fringes if different wavelengths are present.

The wavemeter therefore consists of a Twyman-Green interferometer in which fringes are counted. Normally one counts the fringes for one wavelength AI, N I for a given mirror scan distance and compares it with the number of fringes for the reference wavelength !cr, Nr.

From the equations rnA =2nd, the ratio of the scanned fringes yields the wavelength of the unknown i.e. AI = Nr\N I !cr. If one scans the mirror at a relatively constant speed, the frequency that the fringes pass the detector f= dN/dt can be measured to also yield the same result i.e. AI= fr/ fl !cr.

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The above technique is useful for a CW laser source and is not applicable to pulsed sources. For a pulsed source, scanning is not feasible so that all the required data has to be taken during a single pulse which may range from milliseconds to picoseconds. In this case, the wavelength is determined from the spatial spectral pattern obtain from Fabry-Perot interferometers. In this case two or more interferometers with different spectral resolutions are required . With two interferometers, accuracies of I part in 106 are achievable. However in this experiment, we will only concentrate on the measurement of CW source wavelength.

Figure 39

Figure 40

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7.2 Equipment list

Part Number Description CTY

-

14768-01

SK-08A

SK-25A

RG-23-4

Oscilloscope (not included in kit) 1

Ball Driver Set 1

Screw Kit 1

Screw Kit 1

2'x3' Breadboard 1

BE. Beam Expander Assembly

B-2SA

LC-V

M-40X

MH-2PM

SP-3

SP-4

VPH-3

VPH-4

Base Plate 1

Collimator Module 1

Objective Lens 1

Objective Mount 1

3" Post 1

4" Post 1

3" Post Holder 1

4" Post Holder 1

BS. Beamsplitter Assembly

20B208S.1

U200-A2K

SP-3

VPH-3

2" Beamsplitter 1

Mirror Mount 1

3" Post 1

3" Post Holder 1

CT. Collimation Tester Assembly

200S20

AC-2A

B-2SA

SP-3

VPH-3

2" Collimation Tester 1

Lens Mount 1

Base Plate 1

3" Post 1

3" Post Holder 1

D. Detector Assembly

818-BB-21

B-2SA

BC-5

SP-3

Biased Photodetector 1

Base Plate 1

Base Clamp 1

3" Post 1

VPH-3 3" Post Holder 1

I. Iris Assembly

10-0.5 Iris 2

MCF Flat Carrier 2

MH-2P Iris Mount 2

MSP-3 3" Post 2

MPH-3 3" Post Holder 2

MRL-3 Micro Optical Rail 1

MRL-18M Micro Optical Rail 1

L. Laser Assembly

340-RC Clamp 1

40 Rod 1

ULM-TILT Laser Mount 2

R-30025 1 mW Red HeNe Laser 1

R-30968 Green HeNe Laser 1

MS. Steering Mirror Assembly

10020ER1 1," Mirror 1

COR-1 Cntr Of Rotatn Adaptr 1

P100-P Mirror Mount 1

UPA1 1" Mirror Holder 1

SP-3 3" Post 1

VPH-3 3" Post Holder 1

M1 and M2 Mirror Assemblies

20020ER1 2" Mirror 2

462-X-M Trans Stage 1

CMA-25CC Motorized Actuator 1

8601-10 Cable 1

861 Hand-Held Controller 1

DM-13 Oiff. Micrometer 1

U200-A2K Mirror Mount 2

SP-2 2" Post 1

VPH-2 2" Post Holder 1

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SP-3 3" Post 1

VPH-3 3" Post Holder 1

Screen Assembly

B-2SA Base Plate 1

BC-5 Base Clamp 1

FC-1 Filter Clamp 1

SP-2 2" Post 1

VPH-2 2" Post Holder 1

7.3 Setup

7.3.1 Placement of the Breadboard Place the RG-23-4 breadboard on a flat stable surface. Make sure that there is enough surface area near the breadboard to place the power supply units and other items that need not be mounted .

7.3.2 Laser Setup Mount a 40 Rod on the RG-23-4 breadboard in location L as in Figure 41 Attach a ULM-TILT Laser Mount to a 340-RC Clamp. Slide the 340-RC onto the 40 Rod. Mount the R-30025 laser head in the ULM-TILT mount and align the laser tube so that the polarization plane is perpendicular to the table top ("S" polarization).

7.3.3 Laser Beam Alignment Post mount the Iris Assembly I on the MRL-3 Rail. Tum on the laser, point the beam along the long side of the breadboard and adjust the laser height to 6 inches. Place the 10-0.5 iris directly in front of the laser head (position I. in Figure 41) with its aperture aligned with the laser beam. Move the iris to the other end of the breadboard (position 12 in Figure 41) and adjust tilt and vertical position of the laser on the post to align the beam with the iris aperture. Move the Iris back and forth between positions 11 and 12 to ensure that the beam is parallel to the surface of the breadboard . Once the tilt of the laser is set the height can be varied by the 340-RC clamp and the beam will still be parallel to the surface of the breadboard.

7.3.4 Iris Placement Affix 10-0.5 iris I in front of the laser as shown in Figure 42 and adjust the aperture to just allow the laser beam through. The iris will now be used as a reference for retroreflected beams.

7.3.5 Interferometer Setup Choose one of the setup configurations, Figure 39 or Figure 40 (Figure 40 is an alternative for the setup ofFigure 39 which increases the cross section of the optical windows). Place the 20020ER.1 2" diameter

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mirrors and the 20B20BS.I beamplitter into the U200-A2K mounts and post mount each in place as shown in Figure 40 or Figure 42, to construct the Twyman-Green Interferometer. Mount mirror Ml on 462-X-M stages as in Figure 42. Use 114-20 set screws on the SP-2 and SP-3 posts to connect to U200-A2K mirror mounts. Post mount each interferometer mirror 10" from the beamsplitter.

7.3.6 Interferometer Alignment Center the beam on BS optic and on Ml by adjusting their post heights. Check the beam height in front of mirror Ml. If beam height is not the same before and after the beamsplitter, adjust the tilt of the beam splitter until the beam is horizontal. Place the iris assembly I in front of mirror M2, match the height of the beam by adjusting the beamsplitter and Ml respectively.

7.3.7 Beam Expander Positioning Assemble the beam expander assembly BE and mount in the path of the laser beam as in Figure 43 . Attach the SP-3 post to the B-2SA base and mount the LC-V collimating lens directly onto the B-2SA base. Place the VPH-3 post holder on the breadboard so that when the LC-V is put in place there will be some room left to mount the MAOX objective lens.

Mount the M-40X objective lens directly behind the LC-V. Turn on the laser and adjust the height of the LC-V until the beam is centered on the lens. Insert the M-40X objective lens in its place and align so that the expanding beam is centered on the collimating lens of the LC-V.

7.3.8 Collimation Calibration Place the collimation tester (model No 20QS20) in an AC-2A optics mount (use proper support stud tips in the AC-2A).

NOTE The collimation tester is a wedged plate with its thicker side marked on the edge. It

is desirable to have the thick edge of the plate pointing to the top of the AC-2A.

Place the Collimation Tester Assembly CT at a 45° angle in the path of the expanded beam and look for fringes in the reflection. Adjust the position of the collimating lens in the beam expander until hOlizontal fringes are observed in the reflection. There should be three to five fringes visible in the reflection when fringes are horizontal. At this stage the expanded beam is well collimated.

Place a screen (i.e. a 3" X 5" card) into a FC-I filter clamp and post mount on a SP-2 post, VPH-2 post holder, at the output of the intetferometer and adjust the tilt of the mirrors and beam splitter while looking at the retroreflection on the iris. When the points of light are superimposed on the iris, fringes can be observed on the screen.

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-- 12

Figure 41

I I J

L

I •• MS

MJojiiiiiiiia

Figure 42

~:ll MS

L

Figure 43

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7.4 Procedure

METHOD 1:

1. Fringe pattern- Adjust the tilt of the minors so that 2 to 3 straight fringes are visible. This configuration makes it easy to count fringes and analyze data .

2. Detector placement- Replace the screen with the detector 818-BB-21 so that the fringes pass across the detecting area when minor M2 is moved. Place an adjustable slit in front of the detector (a piece of paper with a small hole poked in it placed directly on the detector will also be sufficient) and adjust the slit width to be less than 114 of the fringe width so that the variation in intensity of moving fringes is easily distinguishable by the detector. Connect the output BNC connector on the detector to channel 1 of the oscilloscope and set the triggering on channell.

3. Actuator assembly- Insert the CMA-25CC actuator into the 462-X-M translation stage and secure in place.

CAUTION The moving platform of the 462-X-M is spring-loaded and wi" snap back against the mount of the adjuster if it is not held in place while the adjuster is being removed . Such an impulse delivered to the stage may cause permanent damage.

Connect the 861 hand held controller to the actuator and to the power supply module provided.

4. Data acquisition- Tum on the 86 1 controller and by pushing the REV button set the actuator position reading to its zero value. Set the velocity of the 861 controller to minimum. Push the FWD button on the controller and hold while looking at the oscilloscope. Adjust the sweep time of the scope to a value where one cycle of the sine wave is displayed. The sine wave on the scope is the variation of the intensi ty of the fringes passing the detecting area of the power meter per unit time. Estimate the period of the wave in seconds and by inverting that value the frequency, fl , of the fringes of the first source is obtained.

5. Second laser source assembly- Use the REV button on the 861 controller to return the actuator to zero reading. Remove the first laser from its mount and remove the ULM-TILT laser mount from the 340-RC clamp. Mount the ULM-TILT laser mount to accommodate the R-30968 green HeNe laser and adjust the optical height and polarization direction of the laser. Use the steering minor MS to direct the beam into the beam expander. It should not be necessary to make adjustments to the beam expander or the iris to obtain the new set of fringes. Make no adjustments to the beam expander. Adjust the tilt of one the minors in the interferometer to obtain one or two visible straight fringes in the field of view. It will be necessary

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to change to more sensitive scale of the oscilloscope. The width of the pinhole (or adjustable slit) may also need adjustment due to the weaker power of the green HeNe laser. Run the 462-X-M stage through another sweep by pressing FWD on the 861 controller and observe the frequency of the fringe movement.

Multiple sets of data- Change the speed to various values and run the same speed for both lasers and compare data.

METHOD 2:

l. Remove the actuator from the 462-X-M translational stage and insert the OM-13 differential micrometer in place.

WARNING The moving platform of the 462-X-M is spring loaded and will snap back against the mount of the adjuster if it is not held in place. Such an impulse delivered to the stage may cause permanent damage.

2. Data acquisition, first source- Move the 462-X-M stage a distance x by turning the micrometer slowly so that the fringes can be counted. The micrometer has high enough resolution to get an accurate count of the fringes. The first count of the fringes recorded is the value N 1. Make sure to record the distance moved.

3. Second laser source- Move the micrometer adjustment back to the zero reading. Replace the first laser with the second laser as indicated in the last section above.

4. Data acquisition, second source- Move the 462-X-M stage the same distance x while counting the fringes. An easy way to accomplish this is to learn to let go of the micrometer adjustment without moving any fringes. This way the last fringe counted will remain visible and you will not lose count of the fringes while checking the distance traveled. Record the number of fringes as N2.

7.5 Expected Resu Its

METHOD 1:

Make a few runs with the wavemeter and calculate each value of the wavelength obtained. Average the values of the wavelengths found and find the standard deviation for the certainty of the wavelength value.

Error Analysis:

Change the tilt of the mirrors and collect the data again to see if the til t of the interferometer will change the number of fringes that travel by. Change the

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7.6

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tilt of the mirrors again so that the fringes move in a different direction when the stage is moved in the same direction.

Figure 44

Some possible directions ofmovement for the fringes for same direction of movement ofone mirror.

METHOD 2:

Make a few runs with the wavemeter and calculate each value of the wavelength obtained. Average the values of the wavelengths found and find the standard deviation for the certainty of the wavelength value.

References [7.1] P. Hariharan, Optical Interferometry, Acaden~ic Press, Sydney (1985).

[7 .2] P. Hariharan, Basics ofInterferometlY, Academic Press, San Diego (1992).

[7.3] F. A. Jenkins and H. E. White, Fundamentals ofOptics, McGraw Hill, New York (1976).

[7.4] E. Hecht, Optics, Addison-Wesley, Reading MA (1987).