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Z-Cut LiNbO₃ Intensity Modulator Cross-Section

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X-Cut LiNbO₃ Intensity Modulator Cross-Section

Features

• Titanium-Indiffused Waveguides
• Low Optical Loss
• Long-Term Bias Stability
• Hermetically Sealed Packaging
• FC/PC Fiber Connectors

Thorlabs manufactures a variety of lithium niobate (LiNbO₃) optical phase, intensity, and I/Q modulators. These high-performance devices are based on titanium-indiffused waveguide technology, offer large bandwidths, and are ideal for developing high-speed modulation systems.

The modulators on this page are fabricated from either X-cut or Z-cut LiNbO₃ (see the example diagrams to the right). X-cut intensity modulators employ a symmetrical design that provides low frequency-chirp in the modulated signal, while Z-cut intensity modulators provide more efficient modulation (i.e., lower V_π or half-wave voltage) at the expense of higher frequency-chirp. Phase modulators are only offered as Z-cut devices because their single optical path does not benefit from the symmetry of the X-cut design. The I/Q modulator fully exploits the advantages of symmetry as an X-cut device.

Z-cut devices are also capable of supporting both the ordinary and extraordinary optical modes, which have different modulation efficiencies. An integrated optical polarizer, positioned before the output port of the device, is included in all of our Z-cut intensity modulators and some of our Z-cut phase modulators as only one mode is desirable for most applications. While most applications benefit from the integrated polarizer, the LN53S-FC, LNP6119, and LNP4217 Z-cut phase modulators are offered for applications where the polarizer is undesirable.

The modulators come with a polarization-maintaining (PM) input fiber pigtail and a single-mode (SM) output fiber pigtail that are terminated with FC/PC connectors. The PM fiber is keyed to the slow axis, which is also aligned to the extraordinary mode of the modulator. Please note that custom options for PM output fiber pigtails and FC/APC connectors are available for all LiNbO₃ modulators. For more information on custom configurations (e.g., fiber type, connectors) and quotes, please contact Tech Support.

All the fiber-coupled LiNbO₃ modulators offered below are compatible with our EO modulator drivers. Our fiber-coupled tunable lasers provide an ideal O-band, C-band, or L-band source for use with these modulators. For all-in-one solutions in high-speed fiber optic test and measurement, we offer reference transmitters, optical transmitter with phase modulators, and calibrated electrical-to-optical converters.

Intensity Modulator Specifications

• These modulators are designed for use at the specified wavelengths. Using the modulator at other wavelengths may cause an increase in the optical loss that is not covered under warranty. In some cases, this loss can be temporary; for instance, the increase in loss caused by shorter wavelengths can usually be reversed by heating the modulator to 80 °C for an hour.
• Extra care should be taken while aligning the polarization state of the optical input to avoid excess optical power from being launched into the ordinary mode, which will be absorbed by the polarizer and can damage the modulator.
• The LNA2322 does not have an internal polarizer. Only extraordinary mode is supported. 

Phase Modulator Specifications

• These modulators are designed for use at the specified wavelengths. Using the modulator at other wavelengths may cause an increase in the optical loss that is not covered under warranty. In some cases, this loss can be temporary; for instance, the increase in loss caused by shorter wavelengths can usually be reversed by heating the modulator to 80 °C for an hour.
• Extra care should be taken while aligning the polarization state of the optical input to avoid excess optical power from being launched into the ordinary mode, which will be absorbed by the polarizer and can damage the modulator.
• These devices do not have an internal polarizer. Both ordinary and extraordinary polarization modes are supported. Optimal modulation is achieved with the extraordinary mode.

I/Q Modulator Specifications

Intensity Modulator Pin Diagrams

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LNA2322 10 GHz Intensity Modulator, X-Cut, Pin Diagram

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LNA2124 10 GHz Intensity Modulator, Z-Cut, Pin Diagram

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LNA6213 40 GHz Intensity Modulator Pin Diagram

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LNA6112 Small Form Factor Housing 40 GHz Intensity Modulator Pin Diagram

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LNLVL-IM-Z Low V_π 40 GHz Intensity Modulator Pin Diagram

Phase Modulator Pin Diagrams

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LN65S-FC and LN53S-FC 10 GHz Phase Modulator Pin Diagram

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LNP6118 and LNP6119 40 GHz Phase Modulator Pin Diagram

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LNP4216 and LNP4217 Low V_π 40 GHz Phase Modulator Pin Diagram

I/Q Modulator Pin Diagram

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LNQ4314 25 GHz I/Q Modulator Pin Diagram

Click for Full Lab Facts Summary

Driving an Electro-Optic Phase Modulator with the Amplified Output of a Function Generator

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Figure 1: Experimental Setup Used to Evaluate Whether a Basic RF Source Built Around a Function Generator Could be Sufficient to Drive a Fiber-Coupled EO Phase Modulator

Thorlabs offers a selection of fiber-coupled electro-optic (EO) modulators, which are ideal for modulating light from fiber-coupled laser sources. Applications frequently require EO modulators to be driven at rates of 1 GHz or higher, which places significant demands on the driving radio frequency (RF) voltage source. We investigated whether it would be possible to use a basic setup built around a function generator to drive a fiber-coupled EO phase modulator. The experimental setup we designed and implemented to test this possibility included instrumentation to record the spectrum of the modulated optical signal. By analyzing the modulated optical spectrum, we confirmed this basic RF source is a viable option for driving a fiber-coupled EO phase modulator. Our approach and results are documented in this Lab Fact.

Experimental Design and Setup

The design of the RF voltage source portion of the setup required first determining the power the RF source should supply to drive the fiber-coupled EO phase modulator. The power requirements were calculated after we made an estimate of the driving voltage needed to achieve the modulation depth desired for this application. Details describing our process for selecting a modulation depth, the relationship between modulation depth and driving voltage, and the calculations we used to estimate the power required from the RF voltage source are included in the Lab Facts document. From our investigations, we determined the power from the function generator alone would not be sufficient for our application. Our solution was to insert a low noise amplifier between the function generator and EO modulator. We also included an electrical low pass filter before the modulator to remove signal distortion that appeared to originate with the function generator. We drove the EO phase modulator with a sinusoidal RF voltage, which imparted a sinusoidal phase modulation on the 1550 nm CW laser signal.

A scanning Fabry-Perot interferometer, whose output was sent to an oscilloscope, was placed after the EO phase modulator and used to measure and monitor the spectrum of the modulated optical signal. It was necessary to use the Fabry-Perot interferometer for this purpose as it has the ability to resolve the very fine spectral features of the phase-modulated optical spectra: at a wavelength of 1550 nm, a frequency difference of 1 GHz is equivalent to a wavelength difference of 0.8 pm. The measured spectra were recorded as functions of scan time. In the Lab Facts document, we describe a straight-forward method to convert from units of Fabry-Perot scan time to units of relative optical frequency. For this work, we estimate Δf = (1.17 GHz/ms)Δt.

Experimental Results

As is described in the Lab Facts document, theory predicts the spectra of our phase modulated optical signals would include sets of symmetric sidebands arranged around the laser carrier peak at frequency f_o. The sidebands are displaced from the laser carrier peak frequency at integer multiples of the modulation frequency f_m (f_o ± Nf_m with N = 1, 2, ...). The relative heights of the sidebands are a function of the modulation depth, which is in turn a function of the peak-to-peak value of the RF driving voltage. Given the modulation depth, the relative amplitudes of the laser carrier peak and modulation sidebands can be calculated. This makes it possible to tailor the power distribution across the various peaks to meet an application's needs. We used the predictive power of this model to confirm our RF source was adequately driving the EO modulator.

The spectra shown in Figures 2 and 4 are representative of the modulation spectra we measured. The theoretical curves in Figure 3 are a function of modulation depth and plot the expected relative powers of the laser carrier peak (solid red curve), first order sidebands (dotted blue curve), second order sidebands (dotted green curve), and third order sidebands (dotted violet curve). The black arrow points to the modulation depth corresponding to the spectrum in Figure 2, and the gray arrow points to the modulation depth corresponding to the spectrum in Figure 4. From our results, we determined our measured and applied modulation frequencies agreed, and we confirmed the spectral power distributions in our optical spectra were consistent with the peak-to-peak driving voltage of the RF source. We conclude that the good agreement between the expected and recorded results validates the use of a basic RF source built around a function generator as a driver for fiber-coupled EO phase modulators. 

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Figure 4: EO Phase Modulator Spectrum When V_pp = 3.63 V
The carrier frequency is f_o; the modulation frequency is f_m = 1 GHz. The X-axis reports the scanning time of the Fabry-Perot interferometer and can be directly related to the signal's relative frequency spectrum.

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Figure 3: Curves Relating the Power in the Carrier and Several Sideband Peaks as A Function of Modulatrion Depth
The 0.44 modulation depth indicated by the black arrow corresponds to Figure 2, and the 0.56 modulation depth indicated by the gray arrow corresponds to Figure 4.

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Figure 2: EO Phase Modulator Spectrum When V_pp = 2.85 V
The carrier frequency is f_o; the modulation frequency is f_m = 1 GHz. The X-axis reports the scanning time of the Fabry-Perot interferometer and can be directly related to the signal's relative frequency spectrum.