Laser Modulation

Drive Electronics
In general, the first application requirements considered in the choice of modulation system components are the information bandwidth and waveform requirement. The driver output voltage achievable is a function of amplifier bandwidth and, while this system parameter is not isolated from others, such as aperture diameter, operating wavelength, etc., it is normally the limiting parameter of the system. Standard Conoptics products include four general purpose drivers: The Models 10, 25, 50 and 100 are dc coupled broadband amplifiers which require an input of 1 volt p-p into 50 ohms for full rated output. Their bandwidths are a function of the modulator used. Each model requires a different electrical configuration in the associated modulator. The Model 10 requires that the modulator be configured as a lumped capacitor. The Model 25 requires a 100 ohm balanced line; the Model 50, a 50 ohm balanced line; and the Model 100, a two segment (4 port) 50 ohm balanced line. The Model 302 is primarily intended for low signal bandwidth, long optical wavelength applications. It also offers cost advantages over higher frequency broadband drivers, especially since, due to its high voltage output, shorter capacitive modulators can be used. The bandwidth of the Model 302 ranges from 150 to 200 kHz depending on the modulator used. Input requirement is 4 volts p-p into 50 ohms. All models include a built in manual bias control.

Optical Modulators
All modulators listed in this data sheet are of the transverse field type, that is, the electric field produced by the applied signal voltage is perpendicular to the optical propagation direction. The voltage swing required by a given modulator at a given operating wavelength to transit between the full off state to the full on state is called the Half Wave Voltage (V½). The transverse field structure allows reduction of V½ by manipulation of the crystal length to aperture ratio to a level achievable by available driver electronics. V½ is roughly proportional to wavelength and long wavelength devices usually require higher length to aperture ratios to accommodate existing driver output levels. Conoptics offers modulators constructed with three different crystal species: Ammonium Dihydrogen Phosphate (ADP), Potassium Dideuterium Phosphate (KD*P), and Lithium Tantalate (LTA). Models 370, 380, and 390 utilize ADP as the active element. The unique feature of these models is the virtual non-existence of piezoelectric resonances. Models belonging to the 360 series utilize LTA. LTA has the lowest intrinsic V½ and the longest wavelength IR cutoff. Furthermore, it has a combination of high refractive index and relatively low dielectric constant which allows modulators to be designed which make full use of the intrinsic driver frequency response. Models in the 360 series exhibit piezoelectric resonances but they are discrete and very narrow. KD*P is used in Model 350 series modulators. In terms of optical transmission bandwidth and driver frequency response utilization, this series falls in between ADP and LTA versions.

Modulator Modifications
Any of the modulators listed here can be used as a phase modulator by simply rotating the input polarization direction by 45°. This procedure makes one of the modulator half segments essentially inactive and doubles V½ (now the voltage required for a 180° phase shift). A factory modification can be made during construction which restores V½ to its original value. This modification precludes use of the device as an intensity modulator, however, and is irreversible.

Auxiliary Components With the exception of 360 Series, modulators used at wavelengths longer than 2000nm, an integral Glan type polarizer (analyzer) is supplied with each model listed here. Operation at longer wavelengths requires polarizers of a different type and may be additional cost items. Other components such as quarter wave plates used in polarization rotators, are also available from Conoptics. The most commonly used auxiliary components are Automatic Bias Controllers (ABC's). The purpose of ABC's is to compensate the long term temperature induced drift of the bias voltage needed to position the applied signal baseline at the desired operating point on the modulator transfer characteristic. Three different versions are available. The first accommodates signal information flows which have a periodic “dead time” such as scanned data or that found in image recorders. Here, a sampling signal is injected by the ABC during the “dead time” and the resulting optical modulation is analyzed to produce an error signal. A feedback loop drives the operating point to the top or bottom of the transfer characteristic, as desired. The second option, used with continuous information flows, such as video disc mastering, samples both the modulated optical output and its reciprocal signal. It averages these samples and produces an error signal which drives the operating point to the midpoint of the transfer characteristic. The third option is similar to the second but is designed to control arbitrary duty cycle digital waveforms. All ABC versions are available with modulation systems incorporating ADP or KD*P modulators and Model 10, 25, 50 and 100 driver electronics. The inherent stability of 360 Series LTA modulators is sufficient in the majority of applications to avoid the need for an ABC. The addition of an ABC to a modulation system requires integration with both the driver electronics and the optical modulator and is a factory installed option.

Modulation Systems
The modulators and drivers listed in this data sheet can be used in various combinations to form high performance, cost effective modulation systems. Table II shows the key performance characteristics of various combinations of standard driver electronics and modulators. The high frequency -3dB points may be limited either by the driver or the modulator. Rise and fall times are normally calculated as 0.35 divided by the -3dB bandwidth but, due to the compression caused by the sine squared transfer characteristic over its full on to off range, the optical rise and fall times of these systems is approximately 20% less. Table 1 Modulator Specifications:

Table 1 Modulator Specifications:

Table 2 Modulation System:

M25D driving 350-160 detected @ 514nm

PHASE MODULATORS

The standard products 350, 360, 370, 380 and 390 series are built as intensity modulators with a polarizer aligned to the crystal axis. These standards can also be used as polarization rotators, voltage variable waveplates or phase modulators. However, when used as a phase modulator only half the cell is active, so the half wave voltage is twice as high as it should be.

Any product can be constructed with all the crystals in-line such that the full cell is active as a phase modulator, but it cannot be used as an intensity modulator (or polarization rotator, variable waveplate).

Please not that the product cannot be re-configured as an intensity modulator once it is built as a phase modulator.

PHASE MODULATOR ALIGNMENT

Linearly polarized light must be passed through the modulator so that that the plane of polarization is orthogonal* to the applied field (see figure 1)

* For 350/370/380/390 Series For 360 Series “plane of polarization is parallel” To perform the alignment of the phase modulator, the optical setup must contain a polarized laser (or an input polarizer (P1) if the laser is unpolarized) and an output polarizer (P2) positioned so that its pass direction is orthogonal to the input (see figure 2)

Align the phase modulator (with the connector vertical or parallel  to the input polarization ) so that the laser beam is centered on the input and the exit crystal faces. Rotate the modulator until a null is observed, after P2. This will align the input polarization parallel to the induced index change. Then remove P2 A suitable modulator support must be provided so that adjustments of the modulator can be made in roll, pitch and yaw. (see figure 3)

RESONANT CIRCUIT E.O. INSTALLATION DIAGRAM

( Click for larger view. » )

MODULATOR MOUNTING ASSEMBLY