2B- Optical Technologies

2B- Optical Technologies
E-Photon One Curriculum 2B- Optical Technologies Coordinator: António Teixeira, Co-Coordinator: K. Heggarty António Teixeira, Paulo André, Rogério Nogueira, Tiago Silveira, Ana Ferreira, Mário Lima, Ferreira da Rocha, João Andrade

E1- 2b Optical technologies
Program Modulators Mach Zehnder Electro-absorption Acoust-optic Filters Fiber Bragg gratings Fabry Perot Mach-Zehnder Isolators Couplers Switches Mechanical Wavelength converters Multiplexers/ Demultiplexers Basic Photonic Measurements Material growth and processing Semiconductor materials Transmission systems performance assessment tools Optical Amplifiers Semiconductor Optical Amplifiers (SOAs) Erbium Doped Fiber Amplifiers (EDFAs) Fiber Amplifiers- Raman Other Amplifiers Emitters Semiconductor Fiber Receivers PIN APD E1- 2b Optical technologies Jan 2006

António Teixeira, Paulo André, Tiago Silveira, Ana Ferreira
Modulators

8. Modulators 1. Modulator Types’ Working Principles Overview 1.1 Electro-absorption modulators (2) 1.2. Acousto-Optic Modulator (2) 1.3. Phase Modulators (1) 1.4. Mach-Zehnder (3) 1.5. Pockels Cells Modulator (1) 1.6. Faraday Effect Based Modulators (1) 2. Modulators 2.1. Modulation (3) 2.2. Modulator’s Types (1) EAM (4) EOM (15) 3. Mach-Zehnder Modulator 3.1. Introduction (3) 3.2. Mach- Zehnder Modulator (3) E1- 2b Optical technologies Jan 2006

8. Modulators 3.3. Electro-optical materials (1) 3.4. Phase modulators (5) 3.5. Mach-Zehnder modulator configurations (1) 3.6. Unbalanced single drive (2) 3.7. Balanced single drive (3) 3.8. Balanced differential drive (4) 3.9. Advanced structures (1) 3.10. Mach-Zehnder modulator characteristics (2) 3.11. Mach-Zehnder modulator design (4) References E1- 2b Optical technologies Jan 2006

Modulator Types - Working Principles

Electro-absorption modulators
A p-n junction when inversely polarized, absorbs light (receiver principle); when not polarized, it presents minimal absorption. depends on the used material’s gap, The p-n junction cut frequency is related to the material gap. only absorbs lower wavelengths than the cut wavelength (with higher energies) since the current modulation is inverse, there is no need to wait for the carrier mobility, because the current will only flow when there is light modulation only depends on the available modulation capacity and not on the device exhibit linear response adapted to analogical modulation “on-state” insertion losses in the order of 9 dB, as an isolated component the device exhibits insignificant losses (1dB) when accoplated to lasers or other devices. E1- 2b Optical technologies Jan 2006

Electro-absorption modulators
The Electrical Field chirp parameter, d(t) given by where data(t) is the data, and power is given by m is the modulation index Used by permission from VPIphotonics, a division of VPIsystems E1- 2b Optical technologies Jan 2006

Acoust-Optic Modulator
When a mechanical vibration is present on a material, causes compression and expansion zones. causes gratings by controlling the vibration frequency and it’s intensity (easily done since the wave is electronically generated) the grating strength can be controlled since the light and sound frequencies are very different (light>>sound, e.g. flight=200THz, fsound=200MHz), so is their speed (light>>sound, e.g. quartz vlight=2E8 e vsound=6E3) which results in wavelengths one or two orders of magnitude apart (v=f, e.g. light=1E-4m e sound=1/3E-5m =>light/sound=30) Important Factors of Bragg refraction the incidence angle is equal to the refraction angle There must be constructive reflection between two optical waves  acoustic wave,  optical wave,  incidence angle E1- 2b Optical technologies Jan 2006

Acoust-Optic Modulator
Common Structures : bragg grating Debye Sears Advantages: - can operate with high powers the refracted signal intensity is: proportional to the acoustic wave intensity can modulate one or more different Wavelengths at the same time the Doppler shift can be used to change the signal's wavelength Disadvantages: relatively high insertion losses require a relatively high drive current the modulation frequency must be inferior to the acoustical wave frequency, therefore it has a low value E1- 2b Optical technologies IBM White Book, pags.249,251 Jan 2006

Phase Modulators When a field is applied to certain materials their refraction index is altered the index’s variation is directly related with the time it takes through the media, and therefore with the signal’s phase Used in coherent modulation, it has appliance on more complex systems E1- 2b Optical technologies IBM White Book, pag.256 Jan 2006

Mach-Zehnder Is the most common type of modulator on telecommunications Can be used at very high modulation frequencies (even for 40Gbit/s there is already market availability) They are based on the delay between tow arms that will induce phase rotations in the order of 180º If there’s no delay, constructive interference will happen If there’s 180º phase delay, there will be destructive interference Normally implemented on integrated technology due to the necessary precision on the guide length The more common material is LiNiO3, but there are other materials E1- 2b Optical technologies IBM White Book, pags.256,257 Jan 2006

Mach-Zehnder The output power depends on the phase difference, , between the two arms of the modulator : extinction ratio considering k=|1|/ |2|, we will have k=-1 – ideal AM modulation (Phase Oposition Modulation) k=0 - Chirp Modulation (only one of the arms is modulated) k>-1 - Phase modulation (Both arms modulated with the same current) Chirp Signal E1- 2b Optical technologies Jan 2006

Mach-Zehnder Alpha Factor : relation between the modulation’s phase and intensity If As an alternative, there are 2 other parameters: simmetry factor Chirp Signal E1- 2b Optical technologies Jan 2006

Pockels Cells Modulator
Some crystals have an electrically controlled birefringence LiNiO3, KDP (NH4H2PO4), ADP (KH2PO4) This effect associated to a polarization filter can be used to get modulation It is rather limited, in account for demanding high modulation tensions (1000V) And for loosing, at the simplest configuration, ½ of the power E1- 2b Optical technologies IBM White Book, pag.259 Jan 2006

Faraday Effect Based Modulators
The modulator is based on the Faraday effect, as mentioned for isolators The difference is that the used material causes variable polarization rotations Is slow and expensive Isn’t used much E1- 2b Optical technologies IBM White Book, pag.260 Jan 2006

Modulators

Modulators Modulation (introduces the information to be transmitted on an optical signal) Efficient Linear Appropriate Bandwidth External - The laser is operated on CW - The laser signal is modulated at the output by a modulator EOM (Electro-optical) EAM (Electro-absorption) - Allows high transmission rates - Coupling Losses can’t be ignored Direct - The laser current is modulated by a digital electronic signal (in amplitude, phase or frequency) - Gives rise to spectrum broadening - “Chirp” E1- 2b Optical technologies Jan 2006

DC RF Direct (laser current) Low Cost External 2.5 to 40 Gb/s DC MOD RF E1- 2b Optical technologies Jan 2006

The optical signal binary modulation can be: Amplitude (ASK) Frequency (FSK) Phase (PSK) The ASK (or OOK) technique is preferred due to its simplicity E1- 2b Optical technologies Jan 2006

Modulator’s Types Nowadays, two types of modulators are used: EAM (electro-absorption modulators) Based on an InP guided wave, whose absorption depends on an applied electrical signal. EOM (electro-optical modulators) Based on a Lithium Niobate guided wave, whose refraction index is altered by an external electrical signal. E1- 2b Optical technologies Jan 2006

EAM Typical structure of an inversely polarized diode Disadvantages: High insertion losses (>10 dB) Narrow spectral window of operation Fixed and non-zero Chirp E1- 2b Optical technologies Jan 2006

Absorption (dB) Wavelength (nm) E1- 2b Optical technologies Jan 2006

Interference Considering two linearly polarized and parallel fields : E1 // E2 The average energy density is: If the fields aren’t orthogonal, there will be a crossed term. + = E1- 2b Optical technologies Jan 2006

Assuming that E1 = E2: I = I0 cos2 (DF/2) The interference can be used to convert the phase variations into intensity variations. E1- 2b Optical technologies Jan 2006

EOM - Manufacture The Lithium Niobate (LiNbO3) is a substrate material used in the manufacture of electro-optical modulators: with an high electro-optical coefficient, that allows the use of low values of modulation tension; With an high optical transparency on the region close to the 1550 nm wavelength; This material, on it’s crystalline form, is usually grown by the Czochralski method; The Wafers obtained this way can be cut accordingly any of the three crystal’s axial directions (x, y and z), given the desired application. E1- 2b Optical technologies Jan 2006

r31 = 6 pm/V r33 = 30 pm/V z (3) y (2) x (1) x (1) Y (2) z (3) X – cut z - cut E1- 2b Optical technologies Jan 2006

- By using the z direction, lesser modulation tension’s modulators can be manufactured - The highest electro-optical tensor component is r33. - The use of crystals with different cut directions, implies the use of appropriate geometries for electrodes and wave guides. Wooten, E.L.; Kissa, K.M.; Yi-Yan, A.; Murphy, E.J.; “A review of lithium niobate modulators for fiber-optic communications systems”, IEEE Journal of Selected Topics in Quantum Electronics, Volume 6, Issue 1, Jan.-Feb. 2000 Page(s): E1- 2b Optical technologies Jan 2006

Waveguides, usually of titanium, are implanted through thermical diffusion in the Lithium Niobate substrate. The electrodes are deposited over a separation layer, usually made of silicon dioxide, placed over the crystal with the purpose of increasing the conformity between the optical signal speed in the waveguide and the modulating electrical signal in the electrode. This layer also allows the rise of the modulator's input impedance and the device’s adaptation to the electrical modulation circuit (typically 50 ). Manufacturing Steps: 70 nm of Ti Deposition 150 nm of SiO2 Deposition 300 nm of Al Deposition Ti diffusion, at 1050º C, for 10 hours Acetone Lift-off E1- 2b Optical technologies Jan 2006

Jan 2006

Refraction Index Profile Refraction Index Depth (μm) E1- 2b Optical technologies Jan 2006

EOM - Structure From the structural point of view, the electro–optical external modulators are based on Mach-Zehnder (MZ) type interferometers Usually, is possible to change the refraction index of both arms. It’s necessary to controle the input optical signal polarization. E1- 2b Optical technologies Jan 2006

Jan 2006

The refraction waveguide index will be given by: The phase difference induced by the electrical signal is: The device’s transference function, assuming that both arms can be modulated is: E1- 2b Optical technologies Jan 2006

Applied Tension Distance between electrodes Electrode Length Usually, the physical parameters are grouped into a constant, Vi, that quantifies the necessary voltage to shift the transmittivity of the modulator between a maximum and minimum value E1- 2b Optical technologies Jan 2006

EOM - Characterization
L = , X = , d = 27m, ne = 2.2, l = 4.0cm, r33 =30.8 pm/V Arm 1 Arm 2 Normalized Transmission Applied Voltage (V) E1- 2b Optical technologies Jan 2006

Extinction Ratio (ER)  chirp parameter Bandwidth: 2.5 Gbit/s – 40 Gbit/s Insertion Losses: dB E1- 2b Optical technologies Jan 2006

Optical Power Instantaneous Frequency Deviation (GHz)
Instant Frequency Deviation Optical Power Instantaneous Frequency Deviation (GHz) Optical Power (mW) Time (ps) E1- 2b Optical technologies Jan 2006

 Arm 1 Modulation Voltage (V) Arm 2 Modulation Voltage (V) Polarization Voltage (V) E1- 2b Optical technologies Jan 2006

ER Arm 2 Modulation Voltage (V) Arm 1 Modulation Voltage (V) Polarization Voltage (V) E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator

Overview Introduction Mach-Zehnder interferometer Applications Mach-Zehnder modulator Electro-optical materials Phase modulators Mach-Zehnder modulator configurations Unbalanced single drive Balanced single drive Balanced differential drive Advanced structures Mach-Zehnder modulator characteristics Mach-Zehnder modulator design References E1- 2b Optical technologies Jan 2006

Intro: Mach-Zehnder interferometer
The operation of a Mach Zehnder Interferometer (MZI) consists in separating an input light beam, applying a transfer function to each of the resultant signals and combining them at the output. The separation and aggregation of the signals is usually performed by two techniques: Two directional couplers, Two Y junctions E1- 2b Optical technologies Jan 2006

Intro: Mach-Zehnder interferometer
The signal at the output is given by: If the transfer functions of the two arms are similar, the output signal is cancelled for directional couplers, similar to the input signal for Y-branch MZI. Y-branch MZI are widely utilized! E1- 2b Optical technologies Jan 2006

Intro: Mach-Zehnder interferometer - Applications
Filters Modulators Wavelength converters Optical regenerators Optical Switching E1- 2b Optical technologies Jan 2006

An optical modulator converts an electrical signal to the optical domain. E1- 2b Optical technologies Jan 2006

If a phase shift (Δ) is inserted between the two arms: Δ=0, The input signal appears at the output. Δ =π, The signal is cancelled and no light appears at the MZI output; Therefore, a phase shift controlled by a data signal can modulate a CW signal. E1- 2b Optical technologies Jan 2006

While a constant phase shift can be obtained if the Mach Zehnder arms have different lengths, in order to allow the dynamic optical phase shift control: electro-optic materials should be employed Allow voltage-induced changes in the refractive index due to the Pockels effect. E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - electro-optic materials
Lithium Niobate (LiNbO3) has a large electro-optic coefficient It depends on the direction of the applied electrical field and the orientation of the LiNbO3 crystal. LiNbO3 waveguides can be made by diffusing titanium into selected regions of the substrate Ease of fabrication ! LiNbO3 is the material of choice for making many active and passive components. E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - phase modulator
The effective mode index of a single waveguide can be changed by applying an external voltage to the contacts parallel to the waveguide This way, the speed of the light beam in the waveguide also changes, provoking a controlled delay in the output. E1- 2b Optical technologies Jan 2006

Electro-optic effect The application of an electrical field results in an Index change Δn, The voltage dependent phase shift is given by: V Applied voltage L Length of the region over which the electrical field is applied de Distance between the electrodes r33 z axis electro-optic coefficient, 30.9 pm/V Γ Accounts for the partial overlap that occurs in practice between the optical and electrical fields (typically 0.5) E1- 2b Optical technologies Jan 2006

The application of a voltage Vπ produces a π phase shift Devices with low Vπ voltage are preferred. This is achieved applying the external electrical field along the crystallographic z axis of the LiNbO3 crystal, because this axis has the largest electro-optic coefficient, r33. The voltage required for π phase shift is obtained by setting Δ = π , V Applied voltage L Length of the region over which the electrical field is applied de Distance between the electrodes r33 z axis electro-optic coefficient, 30.9 pm/V Γ Accounts for the partial overlap that occurs in practice between the optical and electrical fields (typically 0.5) E1- 2b Optical technologies Jan 2006

The signal at the output of the phase modulator is given by: E1- 2b Optical technologies Jan 2006

How to take advantage of the electro-optic coefficient r33? Requirements: The electrical field must be applied along the z-axis of the crystal The light must also be polarized in the z-direction. Two different electrode configurations are used, depending on the crystal orientation: E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator – modulator configurations
Unbalanced single drive Balanced single drive Balanced differential drive Advanced structures are possible, featuring these simple configuration modules E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - unbalanced single-drive
The simplest version is similar to a MZI with a phase modulator in one arm. Not widely utilized because of the high chirp effect. Taylor expansion of the temporal response: E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - unbalanced single-drive
Output power: E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - balanced single-drive
Low or near-zero chirp, usually x-cut or y-cut. The electrode configuration is such that the upper and lower arm undergo an equal but opposite phase change. Separate DC driving can exist for operating point adjustment. E1- 2b Optical technologies Jan 2006

Temporal response: Taylor expansion: Output power: E1- 2b Optical technologies Jan 2006

V(t)=vRF(t)+Vdc E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - balanced differential drive
Widely used in novel modulation schemes Duobinary single sideband … Offers more control over the biasing and chirp conditions of the modulator. Typically z-cut. In order to achieve an opposite phase change in the two arms, a complementary drive has to be applied. E1- 2b Optical technologies Jan 2006

Temporal response: E1- 2b Optical technologies Jan 2006

Response: v1(t)=v2(t)=v(t) v1(t)=-v2(t)=v(t) (push-pull operation) E1- 2b Optical technologies Jan 2006

Mach-Zehnder Modulator - complex MZM based structure
Optical x-cut 4 phase-modulator structure. Employs Y junctions. Optical carrier suppressed Single sideband modulator. Carrier suppression of more than 25dB. Higuma, K.; Hashimoto, Y.; “X-cut LiNbO3 optical SSB modulators”, The 4th Pacific Rim Conference on Lasers and Electro-Optics, CLEO/Pacific Rim 2001, Volume 1, 15-19 July 2001 Page(s):I I-105 vol.1 E1- 2b Optical technologies Jan 2006

MZM Characteristics Insertion loss In the on state, Pout may not equal Pin Extinction ratio [dB] | on-off ratio Measures the suppression of the off state output power, Pmin E1- 2b Optical technologies Jan 2006

MZM Characteristics Frequency chirp Chirp parameter (simplified): Where P(t) and (t) are the output power and phase, respectively. When the modulator is biased at the midpoint of the switching curve and is driven by two identical synchronized signals to produce phase shifts in the two arms of the MZ interferometer, it can be written in the simple form: E1- 2b Optical technologies Jan 2006

Modulator design Design objectives: High modulation efficiency Low applied voltage Low insertion loss High on-off ratio Large modulation bandwidth Negligible or controllable frequency chirp Long lifetime Low cost E1- 2b Optical technologies Jan 2006

Broadband modulator design
LiNbO3 modulators designed for operation at high speeds require a special design A travelling wave approach is common: The electrical radio-frequency (RF) signal propagates on a miniature transmission line in the form of a coplanar waveguide along of the length of the LiNbO3 waveguide, in a way that it overlaps with the optical signal through the device length. E1- 2b Optical technologies Illustration after ref. 8, © 1994 AT&T Jan 2006

Broadband modulator design
Travelling wave: MZI with two Y junctions; The RF signal propagates with the optical field over the entire length in each MZI arm; Several parameters must be optimized: width and height of the electrodes, their spacing, the thickness of the buffer layer used for speed matching … Main problem: The refractive index of LiNbO3 is ~2.2 at optical frequencies but no more than 6 at microwave frequencies A silica buffer layer should be applied: It has a lower refractive index of ~1.9 for RF signals. Its function is to lower the effective index of the transmission line in which the RF signal propagates so that its speed matches that of the optical signal. Losses limit the speed of these modulators: Losses scale with the square root of the modulation frequency! E1- 2b Optical technologies Jan 2006

Broadband modulators Modulation bandwidth Defined as the highest microwave frequency at which the modulated optical power is reduced by 3dB compared with the value obtained at low modulation frequencies. Achievements It is easy to realize modulation bandwidths of 8-10GHz with a proper design. 10GHz modulators exhibit Vπ typically of 5V, and 40GHz modulators of 10-20V. Modulators exceeding 100GHz have been tested in laboratory experiments. E1- 2b Optical technologies Jan 2006

References [1] Govind P. Agrawal, “Lightwave Technology, Components and Devices”, John Wiley & Sons, 2004. [2] Shun Lien Chuang, “Physics of Optoelectronic Devices”, John Wiley & Sons, 1995. [3] Alferness, ” Waveguide Electrooptic Modulators”, IEEE Transactions on Microwave Theory and Techniques, Volume 82, Issue 8, pp – 1137, Aug 1982. [4] Cartledge, J.C.; McKay, R.G.; “Performance of 10 Gb/s lightwave systems using a adjustable chirp optical modulator and linear equalization”, IEEE Photonics Technology Letters, Volume 4, Issue 12, Dec Page(s):1394 – 1397. [5] Higuma, K.; Hashimoto, Y.; Nagata, H.; Oikawa, S.; Izutsu, M.; “X-cut LiNbO3 optical SSB modulators”, The 4th Pacific Rim Conference on Lasers and Electro-Optics, CLEO/Pacific Rim 2001, Volume 1, July 2001 Page(s):I I-105 vol.1 [6] Higuma, K.; Oikawa, S.; Hashimoto, Y.; Nagata, H.; Izutsu, M.; “X-cut lithium niobate optical single- sideband modulator”, Electronics Letters, Volume 37, Issue 8, 12 Apr 2001 Page(s):515 – 516. [7] Wooten, E.L.; Kissa, K.M.; Yi-Yan, A.; Murphy, E.J.; Lafaw, D.A.; Hallemeier, P.F.; Maack, D.; Attanasio, D.V.; Fritz, D.J.; McBrien, G.J.; Bossi, D.E.;“A review of lithium niobate modulators for fiber-optic communications systems”, IEEE Journal of Selected Topics in Quantum Electronics, Volume 6, Issue 1, Jan.-Feb Page(s): [8] AT&T Microelectronics Technical Note “Lithium Niobate Intensity (Amplitude) Modulator”, March 1995 E1- 2b Optical technologies Jan 2006

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