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Free-Space Electro-Optic Modulators
Lithium Niobate Electro-Optic Modulators
Thorlabs free-space electro-optic (EO) amplitude and phase lithium niobate modulators combine our experience with crystal growth and electro-optic materials. Our EO modulators use MgO-doped lithium niobate for high power operation. The modulators have an SMA RF input, which is directly compatible with our HVA200 High Voltage Amplifier (shown below). We offer both broadband DC-coupled and high Q resonant models.
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The required half-wave voltage needed to drive the EO modulators
is a function of the wavelength of the optical signal. The curves labeled
Amplitude Modulators and Phase Modulators describe the broadband
(non-resonant) EO Modulators.
*Please note that AM modulators operating in the
-C4 band (400 - 600 nm) should only be used for AC modulation above a few Hz. MgO-doped lithium niobate can be sensitive to light in this wavelength range, displaying a slow negation to an applied DC field. The more intense the light, the faster the DC field is cancelled. Hence, in practice, a DC voltage cannot be used to bias the modulator at the desired operating point.
DC-Coupled Broadband Modulators
Our broadband EO modulators consist of an EO crystal packaged in a housing optimized for maximum RF performance. The RF drive signal is connected directly to the EO crystal via the SMA RF input. An external RF driver supplies the drive voltage for the desired modulation. The crystal may be modulated from DC up to the frequency limits of the external RF driver. Please see the Specs tab for amplifier requirements. These broadband amplitude and phase modulators are offered with a
High Q Resonant Modulators
Resonant EO Modulators are operated at a fixed frequency, and Thorlabs' design includes a high-Q resonant circuit. When compared with the voltage required to drive broadband EO modulators, the operating voltage of resonant EO modulators is dramatically reduced. Our resonant amplitude and phase modulators operating at 20 MHz are offered with a -C1 AR coating (600 - 900 nm). See the Graphs tab for more information on the reflectance of this coating. Standard and custom AR coating options are available, as well as versions with user-specified resonance frequencies from 0.1 to 100 MHz. Please contact
Driver and Accessories
Our broadband EO phase modulators produce phase shifts from -180° to +180° when the drive voltage varies from -Vπ to +Vπ. While the EO phase modulators require the entire ±Vπ range to fully modulate an optical signal, broadband EO amplitude modulators are able to fully modulate an amplitude signal when the drive voltage varies from 0 to either +Vπ or -Vπ. Please refer to the EO Modulator Intro tab for an illustration.
A standard laboratory function generator is needed in conjunction with a high-voltage amplifier to drive our modulators. Our HVA200 high-voltage amplifier, which features a ±200 V output at 100 mA of continuous current, a 1 MHz bandwidth, and low noise, is ideal for driving our broadband EO modulators when certain operating conditions are met. Generally speaking, EO modulators driven by the HVA200 can fully modulate signals when the required half-wave voltage is ≤200 V. As illustrated in the plot above, this 200 V maximum allows the HVA200 to modulate our EO amplitude modulators and our non-resonant EO phase modulators, at full depth, for wavelengths up to ~620 nm and ~900 nm, respectively. However, the effective voltage range of the amplifier can be extended using the technique described in the Lab Facts tab, which enables our EO amplitude modulators to be fully modulated at wavelengths up to 1000 nm.
Please contact Tech Support if you require more information about driving the resonant or non-resonant broadband EO modulators.
Thorlabs also offers a Glan-Thompson Polarizer mounting adapter, as well as an EO modulator mount for integration into our fiberbench collimation hardware (see below).
These plots show the reflectance of each of our four dielectric coatings for a typical coating run. The shaded region in each graph denotes the spectral range over which the coating is highly transmissive.
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Figure 2: Electro-Optic Amplitude Modulator
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Figure 1: Electro-Optic Phase Modulator
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Figure 3: EO Phase Modulator Output as a Function of Applied Voltage (Listed in Terms of Vπ)
The X marks on the curve correspond to the phase shifts listed in the table above the graph.
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Figure 4: EO Amplitude Modulator Output as a Function of Applied Voltage (Listed in Terms of Vπ)
The X marks on the curve correspond to conditions in the table columns.
Introduction to EO Modulators
Electro-optic (EO) modulators are designed to modulate the phase or intensity (amplitude) of electromagnetic radiation (light) propagating through them. They are used in the Q-switching and mode locking of lasers, the generation of optical pulses, side-band generation, and other applications. EO modulators employ the Pockels effect, which is a linear (to first order) electro-optic phenomenon in which the induced birefringence of certain crystals is proportional to the amplitude of the electric field (voltage) applied across the crystal. Birefringent crystals exhibit one refractive index (n||) for light polarized parallel to the optic axis of the crystal and a different refractive index (n⊥) for light polarized perpendicular to the optic axis. For the crystals used in EO modulators, the optic axis is parallel to the direction of the applied electric field.
Generally speaking, the optical path length (OPL) through a medium is the product of the geometrical length of the path through a medium and the refractive index (nr) of the medium. The refractive index of, and therefore the OPL through, a birefringent crystal is determined by the polarization of the light referenced to the optic axis of the crystal. The OPL through a medium is directly related to the phase accumulated by light travelling through it. When the Pockels effect applies, it is possible to modulate the accumulated phase (phase shift) of the light that propagates through the crystal by modulating the applied voltage. Longer crystals produce larger phase shifts, and the OPL and accumulated phase shift are both functions of the wavelength of the light. EO modulators exploit the changes in the crystal's refractive indices that occur as a response to an applied voltage signal.
EO Phase Modulator
An EO phase modulator applies a voltage-controlled phase shift to the light without affecting its polarization. Figure 1 shows one configuration used to accomplish this: the input light is linearly polarized and aligned with the optic axis of the crystal. As is stated above, the optic axis (parallel to the z-axis in this diagram) is determined by the orientation of the applied electric field. The phase accumulated by the light is a function of the wavelength of the light, the geometric width of the crystal (measured along the y-axis, which is parallel to the direction of propagation), and the refractive index of the optic axis, n||. The modulation of n|| controls the phase modulation of the light; the value of n⊥ is not a factor. Figure 3 lists and plots the phase shift as a function of the applied voltage, which is expressed in terms of Vπ. In the plot, the X marks correspond to the values listed in the table above the graph. Vπ is called the half-wave voltage and is defined as the voltage required to shift the output phase by pi radians.
EO Amplitude Modulator
An example of an EO amplitude modulator is shown in Figure 2. In this configuration, the input light is linearly polarized and oriented at a 45° angle to the crystal's optic axis, which is again determined by the direction of the applied electric field. The input light in this case can be equivalently described as the sum of two equal-amplitude and perpendicularly polarized components: one polarized parallel to the optic axis (along z-axis) and one polarized perpendicular to the optic axis (along the x-axis). These two components accumulate phase differently as they propagate through the crystal, which is a consequence of the birefringence of the crystal; the OPL of the component of light polarized parallel to the optic axis is determined by n||, and the OPL of the perpendicularly polarized component is determined by n⊥. As the applied voltage is varied, the difference between the values of the crystal's two refractive indices varies; therefore, the relative phase shift between the two components also varies.
The polarization of the total electromagnetic field (the sum of the components) at the output of the crystal changes as the two components move in and out of phase with one another. The polarization of the light at the output of the crystal is linear when the two components are in or 180° out of phase, circular when they are π/2 radians out of phase, and elliptical otherwise. The amplitude of the light transmitted by the 45° polarizer is a function of the polarization of the light incident on it; when the applied voltage is modulated, the intensity of the output beam is modulated. The light transmitted by the polarizer, and therefore the EO amplitude modulator, is linearly polarized and oriented parallel to the axis of the polarizer.
The data tabulated in Figure 4 shows the effect of the applied voltage on the phase shift and polarization of the total light field at the output of the crystal. The plot shows the amplitude of the light transmitted by the polarizer as a function of the applied voltage over a modulation range of 2Vπ. (The X marks on the curve correspond to the values listed in the above table.) For the configuration shown in Figure 2, the applied voltage signal must vary from 0 V to Vπ (or from -Vπ to 0 V) to fully modulate the amplitude of input light; in order to capture both a maxima and minima, the modulator must pass through 0 V and either +Vπ or -Vπ. This restriction can be modified when a quarter-wave plate is added to the setup (please see the Lab Facts tab).
Driving EO Modulators
EO modulators may be configured so that the voltage is applied in a direction perpendicular to the propagation of light, as shown in Figures 1 and 2, or in a direction parallel to the propagation of light. An advantage of applying the voltage in the transverse orientation, as shown in these figures, is that it is not necessary for the light to travel through the electrodes. This generally results in better light transmission through the modulator and allows electrode materials that are opaque at the operating wavelength to be used. Additionally, in this transverse configuration, the OPL is proportional to the product of the applied field and crystal length.
Variable high voltage supplies, such as a function generator paired with a high voltage amplifier and delivering hundreds of volts, are typically used to drive free-space EO modulators. As shown by the graph included in the Overview tab, the voltage required to drive the EO modulator increases with the wavelength of the light being modulated. Because of this, at longer wavelengths the voltage needed to achieve full modulation of the optical signal can exceed the voltage output range of the high voltage supply. As an example, the HVA200 high voltage amplifier has a range of ±200 V, which is ideal for driving the broadband EO modulators EO-AM-NR and EO-PM-NR when the wavelength of the light is below approximately 610 and 900 nm, respectively. At higher wavelengths, this amplifier does not supply the voltage needed to achieve full-scale modulation of the optical signals. However, when pairing the HVA200 with EO amplitude modulators such as the EO-AM-NR products, it is possible to extend the available voltage range of the HVA200 by placing a quarter-wave plate before the input of the modulator. This procedure is detailed in the Lab Facts tab.
The driving voltage requirements of fiber-coupled EO modulators are often significantly less than those of free-space EO modulators. In many cases, these modulators have half-wave voltages on the order of 4 V, and one challenge of driving them is locating voltage drivers capable of producing a modulating frequency high enough to fulfill the needs of the application. A basic setup built around a function generator and an amplifier that was successfully used to drive a fiber-coupled EO phase modulator to 1 GHz is described here.
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Figure 1: Electro-Optic Amplitude Modulator Without the Quarter-Wave Plate
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Figure 2: Electro-Optic Amplitude Modulator with a Quarter-Wave Plate at the Input
Pairing an EO Amplitude Modulator with a Quarter-Wave Plate to Reduce Drive Voltage Requirements
Variable high voltage supplies, such as a function generator paired with a high voltage amplifier delivering hundreds of volts, are typically used to drive free-space EO modulators. As shown by the graph included in the Overview tab, the voltage required to drive the EO modulator increases with the wavelength of the light being modulated. Because of this, at longer wavelengths the voltage needed to achieve modulation over the full amplitude or phase range can exceed the output voltage range of the high voltage supply. Please see the EO Modulators tab for more information about EO modulators.
In the case of EO amplitude modulators such as the EO-AM-NR products, placing a quarter-wave plate at the input of the EO amplitude modulator can reduce the maximum voltage necessary to drive the modulator over its full range; when the high voltage supply is the limiting factor, the use of a quarter-wave plate makes available more, or all, of the operational wavelength range of the driven EO amplitude modulator. Figures 1 and 2 show an EO amplitude modulator with and without, respectively, the quarter-wave plate in place. Experiments conducted in our laboratory using the HVA200 high voltage amplifier, which has an output voltage range of ±200 V, and the
The experimental setup used the HRS015 HeNe (633 nm) and LP980-SF15 fiber-pigtailed laser (980 nm) as light sources. A linear polarizer (LPVISB100 or LPNIR100) placed after the source ensured the polarization of the input light was at a 45° angle to the optic axis of the EO-AM-NR-C1 electro-optic amplitude modulator. A second linear polarizer placed at the output of the modulator was aligned parallel to the first. The HVA200, which was used to amplify the 100 kHz triangle wave output by a function generator, produced the variable voltage signal (±200 V maximum) driving the EO amplitude modulator. A triangle waveform was chosen to drive the EO amplitude modulator so that the light transmitted by the modulator would be sinusoidally modulated. Data were taken with and without a quarter-wave plate (WPMQ05M-633 or WPMQ05M-980) placed in front of the EO amplitude modulator. When used, the quarter-wave plate was oriented with its axis parallel to the optic axis of the crystal. (Figure 2 shows the placement and orientation of the quarter-wave plate with respect to the modulator).
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Figure 3: The output of an EO amplitude modulator as a function of applied voltage when the setup does not include a quarter wave plate, diagrammed in Figure 1. The X marks on the curve correspond to conditions in the table columns.
As discussed in the EO Modulators tab and shown in Figure 3, when a quarter-wave plate is not placed at the input of the EO amplitude modulator (Figure 1), the light input to the modulator is linearly polarized and the applied voltage signal must vary from
When a quarter-wave plate is placed in front of the modulator, the light entering the EO amplitude modulator is circularly polarized (as shown in Figure 2). The quarter-wave plate essentially adds an optical bias to the light input to the modulator. With this bias, it is not necessary for the voltage signal to vary between 0 and +Vπ or -Vπ in order fully modulate the amplitude of the input light: a drive voltage range of only -1/2 Vπ to 1/2 Vπ is needed to achieve an amplitude maximum and minimum of the optical signal. The table in Figure 5 illustrates the polarization at the output of EO crystal for a range of drive voltages, and the plot shows the amplitude of the light after the polarizer as a function of drive voltage. When a quarter-wave plate is used and the wavelength of the light source is 633 nm, a drive voltage range of ±110 V is required to fully modulate the EO amplitude modulator. This is well within the range of the HVA200, and the fully modulated optical signal produced using this technique is shown in Figure 6.
By including a quarter-wave plate in the optical setup, the available operating range of the amplifier is effectively doubled. With the bias supplied by the quarter-wave plate, the HVA200 is able to drive the
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Figure 4: This plot shows the distorted modulation response that results when the amplifier is saturated in an attempt to fully modulate a signal whose Vπ is greater than the maximum driving voltage that can be supplied by the amplifier. The wavelength of this optical signal is 633 nm.
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Figure 5: Inserting a quarter-wave plate (QWP) before the EO amplitude modulator, illustrated in Figure 2, shifts the relationship between the output amplitude and the applied voltage. For a bias voltage of 0 V, the output amplitude is halfway between min and max. The X marks on the curve correspond to conditions in the table columns.
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Figure 6: This plot shows the undistorted and fully-modulated 633 nm optical signal achieved by inserting a quarter-wave plate into the optical setup as shown in Figure 2, which makes available the full driving voltage range of the amplifier.
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Amplitude Modulator Crystal Orientation
The electro-optic amplitude modulator (EO-AM) is a Pockels cell type modulator consisting of two matched lithium niobate crystals (see the diagram to the right) packaged in a compact housing with an RF input connector. Applying an electric field to the crystal induces a change in the indices of refraction (both ordinary and extraordinary) giving rise to an electric field dependent birefringence which leads to a change in the polarization state of the optical beam. The EO crystal acts as a variable waveplate with retardance linearly dependent on the applied electric field. By placing a linear polarizer at the exit, the beam intensity through the polarizer varies sinusoidally with linear change in applied voltage.
Please note that AM modulators operating in the -C4 band (400 - 600 nm) should only be used for AC modulation above a few Hz. MgO-doped lithium niobate can be sensitive to light in this wavelength range, displaying a slow negation to an applied DC field. The more intense the light, the faster the DC field is cancelled. Hence, in practice, a DC voltage cannot be used to bias the modulator at the desired operating point. Instead, an adjustable wave plate can be employed to optically bias the modulator to the desired operating point (please see the Lab Facts tab for more information on proper modulator biasing).
Our EO phase modulators provide a variable phase shift on a linearly polarized input beam. The input beam is linearly polarized along the vertical direction which is the Z-axis of the crystal. A voltage at the RF input is applied across the Z-axis electrodes inducing a change in the crystal's extraordinary index of refraction thereby causing a phase shift in the optical signal.
The control signal may be a DC or a time varying RF signal. When the control voltage is a time varying signal, the optical beam undergoes frequency modulation whereby some of the energy at the fundamental frequency is converted into sidebands separated from the fundamental frequency by the integer multiples of the modulating frequency. The amount of energy converted into sidebands is determined by the depth of modulation. The graph to the right shows a plot of relative sideband strength as a function of depth of modulation.
Our resonant EO phase modulators feature a high Q, and can be driven by a standard laboratory function generator. The high Q resonant tank circuit located inside the modulator boosts the low level RF input voltage from a standard function generator to the high voltage needed to achieve full depth of modulation. This results in a Half-Wave Drive Voltage of only 15 V at 633 nm.
Resonant modulators are offered in both Phase- and Amplitude-modulating versions, operating at 20 MHz with a 1 MHz bandwidth. They feature a C1 coating for use from 600 to 900 nm. Custom versions are also available, with user-specified resonant frequencies from 0.1 to 100 MHz and a variety of AR coatings.
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EO-PMT Polarizer Mounting Adapter
The EO-PMT allows for easy interfacing of a Glan Thompson polarizer (GTH5M) to our Electro-Optic Modulators. The single mounting point allows the mount to swivel the polarizer into and out of the light path; a useful feature that allows for alignment and adjustment of the electro-optic modulators. The EO-GTH5M packages the Glan Thompson polarizer (GTH5M) with the mounting adapter.
Our EO Modulators Can Be Mounted Using the PY005 5-Axis Stage, FT-EOMA Bracket and PY005A1 Base (Left) for a 2" (50.8 mm) Beam Height or Directly to the PY005 Stage (Right)
The PY005 is a compact stage with five degrees of freedom. Two actuators adjust the yaw and Y axes, two actuators adjust the pitch and Z axes, and a single actuator adjusts the X axis. Two F19SC1 locking collars are epoxied onto the yaw and Y-axis actuators, acting as hard stops so these actuators cannot be overdriven. The pitch, X-axis, and Z-axis actuators bottom out inside bores, which prevents them from being overdriven. Please note that using F19SC1 collars with the pitch, X-axis, and Z-axis actuators is not recommended, as it will reduce the overall travel range of the stage. Click here for full information about the PY005 stage.
An EO modulator can be mounted directly to the PY005 using the #8 counterbore accessible from the bottom of the stage (shown to the far right). This counterbore is accessible even when the PY005A1 base is attached to the stage. Additionally, the EO modulator can be mounted to the stage using the FT-EOMA adapter bracket (shown to the immediate right). When the FT-EOMA is used to mount the modulator and the PY005A1 mounting base is attached to the bottom of the stage, the optical axis of the EO modulator is positioned at a nominal height of 2" (50.8 mm) from the optical table.
The FT-EOMA adapter bracket can also be used to mount a Modulator onto a FiberBench. The length of the bench needs to be at least 70 mm to mount the modulator. Incorporation of the optional EO Modulator optic mount and polarizer (EO-GTH5M) will require a longer FiberBench. The Linear Polarizer modules are well suited for use with an EO Modulator in a FiberBench System. The application picture to the right shows an EO Modulator with an FT-38X100 Multi-Axis FiberBench, one Linear Polarizer, and two FiberPorts.