英文原文1.5 Experimental SetupDue to the many concepts and variations involved in performing the experiments in this project and also because of their introductory nature, Project 1 will very likely be the most time consuming project in this kit. This project may require as much as 9 hours to complete. We recommend that you perform the experiments in two or more laboratory sessions. For example, power and astigmatic distance characteristics may be examined in the first session and the last two experiments (frequency and amplitude characteristics) may be performed in the second session.A Note of CautionAll of the above comments refer to single-mode operation of the laser which is a very fragile device with respect to reflections and operating point. One must ensure that before performing measurements the laser is indeed operating single-mode. This can be realized if a single, broad fringe pattern is obtained or equivalently a good sinusoidal output is obtained from the Michelson interferometer as the path imbalance is scanned. If this is not the case, the laser is probably operating multimode and its current should be adjusted. If single-mode operation cannot be achieved by adjusting the current, then reflections may be driving the laser multimode, in which case the setup should be adjusted to minimize reflections. If still not operating single-mode, the laser diode may have been damaged and may need to be replaced.WarningThe lasers provided in this project kit emit invisible radiation that can damage the human eye. It is essential that you avoid direct eye exposure to the laser beam. We recommend the use of protective eyewear designed for use at the laser wavelength of 780 nm.Read the Safety sections in the Laser Diode Driver Operating Manual and in the laser diode section of Component Handling and Assembly (Appendix A) before proceeding.1.5.1 Semiconductor Diode Laser Power Characteristics1.Assemble the laser mount assembly (LMA-I) and connect the laser to its power supply. We will first collimate the light beam. Connect the laser beam to a video monitor and image the laser beam on a white sheet of paper held about two to tencentimeters from the laser assembly. Slowly increase the drive current to the laser and observe the spot on the white card. The threshold drive current rating of the laser is supplied with each laser. Increase the current to about 10-20 mA over the threshold value.With the infrared imager or infrared sensor card, observe the spot on the card and adjust the collimator lens position in the laser assembly LMA-I to obtain a bright spot on the card. Move the card to about 30 to 60 centimeters from the lens and adjust the lens position relative to the laser to obtain a spot where size does not vary strongly with the position of the white card. When the spot size remains roughly constant as the card is moved closer or further from the laser, the output can be considered collimated. Alternatively, the laser beam may be collimated by focusing it at a distance as far away as possible. Protect fellow co-workers from accidental exposure to the laser beam.2.Place an 818-SL detector on a post mount (assembly M818) and adjust its position so that its active area is in the center of the beam. There should be adequate optical power falling on the detector to get a strong signal. Connect the photodetector to the power meter (815). Reduce the background lighting (room lights) so that the signal being detected is only from the laser. Reduce the drive current to a few milliamperes below threshold and, again, check to see that room light is not the dominant signal at the detector by blocking the laser light.3. Increase the current and record the output of the detector as a function of laser drive current. You should obtain a curve similar to Figure 1.2. If desired, the diode temperature may also be varied to observe the effects of temperature on threshold current. When examining laser diode temperature characteristics, the laser diode driver should be operated in the constant current mode as a safeguard against excessive currents that damage the diode laser. Note that as the diode temperature is reduced, the threshold decreases. Start all measurements with the diode current off to prevent damage to the laser by preventing drive currents too high above threshold. To prevent destruction of the laser, do not exceed the stated maximum drive current of the laser.1.5.2 Astigmatic Distance CharacteristicsThe laser diode astigmatic distance is determined as follows. A lens is used to focus the laser beam at a convenient distance. A razor blade is, then, incrementally moved across the beam to obtain data for total optical power passing the razor edge vs. the razor blade position. A plot of this data produces an integrated power profile of the laser beam (Figure 1.9a) which through differentiation exposes the actual power profile (Figure 1.9b) which, in turn, permits determination of the beam diameter (W).A beam diameter profile is obtained by measuring the beam diameter while varying the laser position. Figure 1.9c illustrates the two beam diameter profiles of interest: one for razor edge travel in the direction perpendicular to the laser diode junction plane and the other for travel in the direction parallel to the junction plane. The astigmatic distance for a laser diode is the displacement between the minima of these two profiles. This method is known as the knife edge technique.1. Assemble the components shown in Figure 1.8 with the collimator lens (LC), in the rotational stage assembly (RSA-I), placed roughly 1 centimeter away from the laser. The beam should travel along the optic axis of the lens. This is the same lens used in collimating the laser in the previous setup. The approximate placement of all the components are shown in the figure. Make sure that the plane of the diode junction(xz plane in Figure 1.1) is parallel with the table surface.2. Due to the asymmetric divergence of the light, the cross-section of the beam leaving the laser and, further, past the spherical lens is elliptical. The beam, thus, has two distinct focal points, one in the plane parallel and the other in the plane perpendicular to the laser diode junction. There is a point between the two focal points where the beam cross-section is circular. With the infrared imager and a white card, roughly determine the position where the beam cross-section is circular.Figure 1.9 – Procedure for finding astigmatic distance.3. Adjust the laser diode to lens distance such that the razor blades are located in the xy plane where the beam cross-section is circular.4. Move the laser diode away from the lens until minimum beam waist is reached at the plane of razor blades. Now, move the laser diode about 200 µm further away from the lens.5. Move razor blade 1 in the x direction across the beam through the beam spread θx and record the x position and detected intensity at each increment (≤100 µm increments). The expected output is shown in Figure 1.9. The derivative of this curve yields the intensity profile of the beam in the x direction from which the beam diameter is determined.6. Repeat with razor blade 2 for θy in the y direction.7. Move the laser closer to the lens in increments (≤50 µm) through a total of at least than 500µm. Repeat Steps 5 and 6 at each z increment, recording the z position.8. Using the collected data, determine the beam intensity profiles in the x and y directions as a function of the lens position z. This is done by differentiating each data set with respect to position. Then, calculate the beam diameter and plot as a function of z. The difference in z for the minimum in θx and θy is the astigmatic distance of the laser diode. Use of computer software, especially in differentiating the data, is highly recommended.If the laser junction is not parallel to the table surface, then for each measurement above, you will obtain an admixture of the two beam divergences and the measurement will become imprecise. If the laser is oriented at 45° to the surface of the table, the astigmatic distance will be zero.Different laser structures will have different angular beam divergences and, thus, different astigmatic distances. If you have access to several different laser types (gain guided, index guided), it may be instructive to characterize their astigmatic distances.1.5.3 Frequency Characteristics of Diode LasersIn order to study frequency characteristics of a diode laser, we will employ a Michelson interferometer to convert frequency variations into intensity variations. An experimental setup for examining frequency and, also, amplitude characteristics of a laser source is illustrated in Figure 1.10.1. In this experiment, it is very possible that light may be coupled back into the laser, thereby, destabilizing it. An optical isolator, therefore, will be required to minimize feedback into the laser. A simple isolator will be constructed using a polarizing beam splitter cube and a quarterwave plate. We orient the quarterwave plate such that the linearly polarized light from the polarizer is incident at 45° to the principal axes of the quarterwave plate so that light emerging from the quarterwave plate is circularly polarized. Reflections change left-circular polarized light into right-circular or vice versa so that reflected light returning through the quarterwave plate will be linearly polarized and 90° rotated with respect to the polarizer transmission axis. The polarizer, then, greatly attenuates the return beam.In assembling the isolator, make sure that the laser junction (xz plane in Figure 1.1) is parallel to the surface of the table (the notch on the laser diode case points upward) and the beam is collimated by the lens. The laser beam should be parallel to the surface of the optical table. Set the polarizer and quarterwave (λ/4) plate in place.Pla ce a mirror after the λ/4 plate and rotate the λ/4 plate so that maximum rejected signal is obtained from the rejection port of the polarizing beam splitter cube as shown in Figure 1.11. When this signal is maximized, the feedback to the laser should be at a minimum.2. Construct the Michelson interferometer as shown in Figure 1.12. Place the beam steering assembly (BSA-II) on the optical table and use the reflected beam from the mirror to adjust the quarterwave plate orientation. Set the cube mount (CM) on the optical breadboard, place a double sided piece of adhesive tape on the mount, and put the nonpolarizing beam splitter cube (05BC16NP.6) on the adhesive tape. Next, place the other beam steering assembly (BSA-I) and the detector mount (M818BB) in location and adjust the mirrors so that the beams reflected from the two mirrors overlap at the detector.When long path length measurements are made, the interferometer signal will decrease or disappear if the laser coherence length is less than the two way interferometer path imbalance. If this is the case, shorten the interferometer until the signal reappears. If this does not work, then check the laser for single-mode operation by looking for the fringe pattern on a card or by scanning the piezoelectric transducer block (PZB)in BSA-II and monitoring the detector output which should be sinusoidal with PZB scan distance. If the laser does not appear to be operating single-mode, realign the isolator and/or change the laser operating point by varying the bias current. Additionally, to ensure single-mode operation, the laser should be DC biased above threshold before applying AC modulation. Overdriving the laser can also force it into multimode operation.3. The Michelson interferometer has the property that depending on the position of the mirrors, light may strongly couple back toward the laser input port. In order to further reduce the feed-back, slightly tilt the mirrors as illustrated in Figure 1.13. If still unable to obtain single-mode operation, replace the laser diode.4. Place a white card in front of the detector and observe the fringe pattern with the infrared imager. Slightly adjust the mirrors to obtain the best fringe pattern. Try to obtain one broad fringe.5. Position the detector at the center of the fringe pattern so that it intercepts no more than a portion of the centered peak.6. By applying a voltage to the piezoelectric transducer block attached to the mirror (part PZB) in one arm of the interferometer (i.e. BSA-II), maximize the output intensity. The output should be stable over a time period of a minute or so. If it is not, verify that all components are rigidly mounted. If they are, then room air currents may be destabilizing the setup. In this case, place a box (cardboard will do) over the setup to prevent air currents from disturbing the interferometer setup.7.Place the interferometer in quadrature (point of maximum sensitivity between maximum and minimum outputs of the interferometer) by varying the voltage on the PZB.8. The output signal of the interferometer due to frequency shifting of the laser is given by ∆I∝∆φ = 2π/c ∆L ∆ν where ∆L is the difference in path length between the two arms of the interferometer and ∆ν is the frequency sweep of the laser that is induced by applying a current modulation. Remember that in a Michelson interferometer ∆L is twice the physical difference in length between the arms since light traverses this length difference in both directions. ∆L values of 3-20 cm represent convenient length differ ences with the larger ∆L yielding higher output signals.Before we apply a current modulation to the laser, note that the interferometer output signal, ∆I, should be made larger than the detector or laser noise levels by proper choice of ∆L and current mo dulation amplitude di. Also recall from Section 1.3that when the diode current is modulated so is the laser intensity as well as its frequency. We can measure the laser intensity modulation by blocking one arm of the interferometer. This eliminates interference and enables measurement of the intensity modulation depth. We, then, subtract this value from the total interferometer output to determine the true dI/di due to frequency modulation. Apply a low frequency, small current modulation to the laser diode. Note that when the proper range is being observed15mA 10didv v 1--= and1mA 2.0didI I 1-= for the amplitude change only.Recallingi v L c 2di d di dI ∆∆∆=∆∝πφ）（ ,15mA 10~didI Lv 2c --∆π, or15-mA 10L K 2~di dI -∆λπ where K is a detector response constant determined by varying ∆L.9. With the interferometer and detection system properly adjusted, vary the drive frequency of the laser and obtain the frequency response of the laser (Figure 1.4 or1.10a).You will need to record two sets of data: (i) the modulation depth of the interferometer output as a function of frequency, and (ii) the laser intensity modulation depth. The difference of the two sets of collected data will provide an estimate of the actual dI/di due to frequency modulation. Also note that if the current modulation is sufficiently small and the path mismatch sufficiently large, the laser intensity modulation may be negligible. You may need to actively keep theinterferometer in quadrature by adjusting the PZB voltage.Make any necessary function generator amplitude adjustments to keep the current modulation depth of the laser constant as you vary the frequency. This is because the function generator/driver combination may not have a flat frequency response. The effect of this is that the current modulation depth di is not constant and varies with frequency. So to avoid unnecessary calculations, monitor the current modulation depth by connecting the LASER MONITOR connector on the laser diode driver system to an oscilloscope and keep the modulation depth constant by adjusting the amplitude of the applied sinusoidal wave as a function of frequency. Record the frequency for your laser at which the thermal contribution to dν/di begins to become negligible and dν/di drops off (see Section 1.3).10. Keeping the above equations in mind, we will, now, measure the FM chirp characteristics of the laser. At a constant current modulation frequency (choose a modulation frequency where dν/di varies rapidly, i.e. where the slope of your graph from Step 9, which should be similar to Figure 1.10a, is maximum), vary the current modulation depth di for different laser bias levels and derive a curve such as the one in Figure1.10b.The output dν should not vary significantly except around threshold and at high currents.CautionDo not exceed the specified drive currents/output power ratings of the diode or it may be damaged.11. The phase noise characteristic behavior (Section1.4) as a function of interferometer path length imbalance ∆L may be determined by ind ucing phase noise through application of laser current modulation. Make sure that the interferometer is in quadrature.Set the laser diode current above threshold, apply a small current modulation, and fix the modulation frequency at a desired value. Convenient frequencies may include 50 Hz, 2 kHz, and 50 kHz (see Reference 1.5). Monitor the detector output with a spectrum analyzer or an oscilloscope and record the peak-to-peak output intensity at interferometer quadrature. You may accomplish this by manually sweeping the PZB voltage to cause a minimum of π/2 phase shift, recording the maximum peak-to-peak intensity as a function of path length imbalance. It is important to ensure that instrument noise is below the signal levels expected and it is assumed that single-mode operation of the laser is maintained. Curves similar to Figure 1.10c should be obtained.1.5.4 Amplitude Characteristics of Diode LasersThe measurements of the intensity characteristics are taken by placing the detector before the interferometer as in Figure 1.10 or by blocking one mirror in the interferometer. Again, the laser must be operated single-moded with minimum feedback or the noise level and functionality will drastically change. The relative intensity noise (RIN) is defined as 20log(dI/I) where dI is the RMS intensity fluctuations so that for dI~10-4 , the RIN is -80 dB. Normally, these measurements are made with a spectrum analyzer and a 1 Hz bandwidth.When making RIN measurements, electronic and photodetector shot noise must be below the RIN levels. (OPTIONAL) You may determine the shot noise using an incoherent source (e.g. lamp) with an intensity level similar to that of the laser. The resultant frequency spectrum of noise with the light source excited gives a measure of the shot noise level which should be adjusted to be at least 10 dB greater than electronic noise levels. The measured shot noise should be checked with Equation0.47.1. Vary the laser drive current from below threshold through and above the threshold and record the laser output power and intensity noise at a desired frequency using a spectrum analyzer. When you calculate the RIN, assuming that shot and electronic noises are below the RIN level, a plot similar to that presented in Figure 1.10d should be obtained. In most cases, for single-mode operation, the noise peaks at threshold. The shape of the noise curve may vary if the laser is modulated, if it becomes multi-modal, or if the side-mode suppression on a nominally single-mode laser is not adequate (< 20 dB).2. It is instructive to operate the laser with modulation signals of varying depth and/or degrading the isolator performance by rotating the λ/4 plate to increase feedback to the laser. This will illustrate noise properties for various feedback conditions which are important to subsequent sensor and communication experiments. RINs of less than -150dB and -120dB are required for television broadcast signals and sensitive interferometric sensors, respectively.3. The intensity noise of diode lasers has a 1/f characteristic (performance is degraded as the frequency is lowered). With the laser above threshold and the photodetector connected to a spectrum analyzer, determine the RIN as a function of modulation frequency. The response shown in Figure 1.10e should be obtained where the noise becomes white (flat with frequency) starting somewhere between 100 kHz and 1 MHz for typical lasers.NOTE: The Michelson interferometer setup used in this project will again be used in Project3. It may, therefore, save time to proceed directly to Project3 before completing characterization of diode lasers in Project2.。