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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 4 — Feb. 25, 2013
  • pp: 4396–4404
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Diminishing relative jitter in electrooptic sampling of active mm-wave and THz circuits

M. Jamshidifar and P. Haring Bolívar  »View Author Affiliations


Optics Express, Vol. 21, Issue 4, pp. 4396-4404 (2013)
http://dx.doi.org/10.1364/OE.21.004396


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Abstract

In this work a novel approach in synchronization of electrooptic sampling systems for the ultra-broadband characterization of active mm-wave and THz devices is presented. The relative time jitter between sampled circuit and probing electrooptic head is eliminated by using a femtosecond laser system both as the generator of CW driving the device under test as well as the impulsively probing element. Previous ultra-broadband approaches were applicable to passive components driven by THz impulses, only. The presented system is more generally applicable to active mm-wave and THz components driven by conventional CW electronic sources. Broadband analysis on silicon nonlinear transmission line elements up to a frequency of 300 GHz is presented in order to illustrate the capabilities of the concept.

© 2013 OSA

1. Introduction

Since the early 80s [1

1. J. A. Valdmanis, G. A. Mourou, and C. W. Gabel, “Picosecond electro-optic sampling system,” Appl. Phys. Lett. 41(3), 211–212 (1982).

], electrooptic sampling (EOS) emerged as an advanced solution to characterize high frequency structures and devices with an enormous bandwidth. Although electronic measurement systems like microwave network analyzers are the prevalent methods to characterize high frequency components, EOS remains highly attractive given its ultra-wideband device characterization and freely-positionable near field sensing capabilities. The bandwidth of EOS is limited by the sampling laser pulse width and dispersion in the sampling head only, enabling analysis far beyond electrically measurable frequencies. Electrooptic detection of electric fields up to frequencies of several THz has been demonstrated reaching more than 1 THz in near field [2

2. H. -M. Heiliger, M. Nagel, H. G. Roskos, and H. Kurz,” Thin-film microstrip lines for mm and sub-mm-wave on-chip interconnects,” IEEE MTT-S Int. Microwave Symp. Dig.421–424 (1997).

], and more than 100 THz in free space configurations [3

3. C. Kübler, R. Huber, S. Tübel, and A. Leitenstorfer, “Ultrabroadband detecetion of multi-terahertz field transients with GaSe electro-optic sensors: approaching the near infrared,” Appl. Phys. Lett. 85, 3360–3362 (2004).

]. Such techniques can be combined with photoconductive switches, in order to excite and analyze passive electronic components impulsively. However, this big potential can strongly be degraded for the characterization of active or nonlinear electronic components as in this case impulsive excitation is not applicable. In this case the input signal of a device under test (DUT) is driven by a microwave synthesizer and the bandwidth is severely limited by the relative timing jitter between EO laser sampling and microwave driving unit [4

4. K. S. Giboney, S. T. Allen, M. J. W. Rodwell, and J. E. Bowers, “Picoseconds measurements by free-running electro-optic sampling,” IEEE Photon. Technol. Lett. 6(11), 1353–1355 (1994). [CrossRef]

]. Regardless of whether the laser is synchronized to the microwave source or vice versa [5

5. T. Pfeifer, H. M. Heiliger, T. Löffler, C. Ohlhoff, C. Meyer, G. Lüpke, H. G. Roskos, and H. Kurz, “Optoelectronic on-chip characterization of ultrafast electric devices: measurements techniques and applications,” IEEE J. Sel. Top. Quantum Electron. 2(3), 586–604 (1996). [CrossRef]

,6

6. K. Yang, L. P. B. Katehi, and J. F. Whitaker, “Electric field mapping system using an opticl-fiber-based electrooptic probe,” IEEE Microw. Wirel. Compon. Lett. 11(4), 164–166 (2001). [CrossRef]

], fast timing jitter cannot fully be eliminated given the required finite time constant of the synchronization loop (typically a PLL). Presently, EOS measurements of active or nonlinear mm-wave (sub THz) circuits are restricted to a frequency of 140 GHz only (measurement with lock-in amplifier in [7

7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

]). Two approaches have been adapted to date [5

5. T. Pfeifer, H. M. Heiliger, T. Löffler, C. Ohlhoff, C. Meyer, G. Lüpke, H. G. Roskos, and H. Kurz, “Optoelectronic on-chip characterization of ultrafast electric devices: measurements techniques and applications,” IEEE J. Sel. Top. Quantum Electron. 2(3), 586–604 (1996). [CrossRef]

,6

6. K. Yang, L. P. B. Katehi, and J. F. Whitaker, “Electric field mapping system using an opticl-fiber-based electrooptic probe,” IEEE Microw. Wirel. Compon. Lett. 11(4), 164–166 (2001). [CrossRef]

] which are both fundamentally based on a PLL system. In one alternative [5

5. T. Pfeifer, H. M. Heiliger, T. Löffler, C. Ohlhoff, C. Meyer, G. Lüpke, H. G. Roskos, and H. Kurz, “Optoelectronic on-chip characterization of ultrafast electric devices: measurements techniques and applications,” IEEE J. Sel. Top. Quantum Electron. 2(3), 586–604 (1996). [CrossRef]

], microwave generator and laser are configured as master - slave respectively (MM-LS configuration). In this system feedback error signals modify the laser repetition frequency, by adjusting laser cavity length, in order to compensate its deviation from a microwave triggered signal. A finite intermediate frequency IF is chosen, in order to minimize the influence of noise in the measurement. Unfortunately this MM-LS scheme is unable to fully cope with fast-term jitter due to either slow mechanical adjustment of the laser cavity or noise in the feedback error signal which can unlock the PLL. Another approach is to extracting a synchronization signal from laser repetition rate [6

6. K. Yang, L. P. B. Katehi, and J. F. Whitaker, “Electric field mapping system using an opticl-fiber-based electrooptic probe,” IEEE Microw. Wirel. Compon. Lett. 11(4), 164–166 (2001). [CrossRef]

,7

7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

] and feeding it into the external reference of microwave generator. This laser master – microwave slave (LM-MS) method also faces limitation given to slowness of the PLL synchronization which induces a large time constant while dealing with the ripples and noise in the synchronization signal. Both approaches are incapable to deal with fast timescale jitter within the time constant of the PLL loops, severely limiting the measurement bandwidth. In this paper, we present a new synchronization approach which allows circumventing this problem completely, by conceiving a direct synchronization between laser and mm-wave signals.

2. Experimental setup with new synchronization technique

A simplified schematic diagram of a conventional MM-LS setup is shown in Fig. 1 (a)
Fig. 1 Electrooptic sampling (a) old setup or a conventional MM-LS (LM-Ms) setup with PLL synchronization and microwave source to drive the DUT (b) the LM-LS setup with new synchronization technique, whereby the laser itself generates the microwave signal directly, without a PLL loop.(c) a detailed schematic of the LM-LS setup (b) and the mechanism of generating microwave signal from laser to drive DUT.
(see [7

7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

] for more details). In such a traditional system a microwave signal generator injects a signal into an ultrafast DUT and the operation of the DUT is analyzed with an EO probe which sampling the electric field on the circuit with femtosecond laser pulses. In this paper we assess the capabilities of such a system by investigating a 65nm CMOS nonlinear transmission line (NLTL) [7

7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

] which generates a broad comb of frequencies in the mm-wave and THz ranges. As mentioned earlier, due to imperfection of synchronization, relative time jitter between microwave generator and femtosecond laser pulse, can reduce the measurement bandwidth. In this case, as will be shown later, although the circuit contains frequency components up to a frequency of 300 GHz, the signal is severely influenced by fast timing jitter, which reduces the measured bandwidth to 150 GHz, only. Generally in such a setup a lock-in amplifier is used to amplify the EO signals and reduce the noise in the measurements, but the presence of the jitter in the measured signal leads to IF signals which have a line width broader than the locking measurement time constant, leading to measurement errors [7

7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

]. This is particularly problematic, as the most attractive and valuable data at higher harmonics of the NLTL signal deteriorates due to increasing influence of jitter with respect to the harmonic number. To rationalize this effect one can imagine that 1% of jitter in fundamental signal can be magnified to 10% at 10th harmonic. In the analyzed DUT even higher harmonics have to be detected.

Alternatively, in order to reduce the influence of jitter we introduce a setup with a new synchronization method: The setup is shown in Fig. 1 (b). In contrast to conventional MM-LS or LM-MS systems, the main difference is that the laser is the only source in the setup responsible both for pumping the microwave signal into the DUT and electrooptic probing. This scheme eliminates the use of a microwave synthesizer. In order to excite the circuit, as seen in Fig. 1 (b), a beam of the laser is used to illuminate a fast photodiode, generating an electronic signal with a comb of harmonics of the laser repetition rate (75 MHz). By using a tuned band pass filter at a harmonic of 10 GHz, the microwave excitation signal is selected and amplified to drive the NLTL. The great advantage of this approach is that a slow PLL synchronization loop can fully be eliminated. For frequency selection a rather sharp cavity filter with center frequency of about fc = 10 GHz and quality factor of Q = 200 is used. The filter can select the 134th harmonic of a typical 75 MHz laser repetition rate and rejects the lower and upper harmonics (fc ± 75 MHz) with approximately 55 dB of attenuation. The spectrum of this signal is shown in Fig. 2
Fig. 2 Measured microwave signal generated from femto-second laser pulse in comparison to a signal of Rohde&Schwarz® SMF 100A low phase noise microwave synthesizer. The phase noise of the signals is close to each other.
. Hence the laser cavity length and consequently laser repetition rate can precisely (1 Hz resolution) be adjusted in the range of 75 to 76 MHz, therefore there is the possibility to utilize an arbitrary tuned filter and adjusting the laser to resonate at a flexibly definable center frequency. The level of this laser generated microwave signal, immediately after filter, is measured at −55 dBm using Tektronix 2755P spectrum analyzer. The signal has been amplified by a LNA and sort of cascaded amplifiers to level of 0 dBm. This signal is delivered into the NLTL after one more amplification stage with 18 dB of gain (see Fig. 1(c)). As this system is self-referencing the circuit excitation path and circuit sampling path to the same laser source, and the temporal paths along both can be chose to be similar or even identical, the influence of jitter can much more effectively be minimized. The only disadvantage is the additional burden of having to modify components when other excitation frequencies have to be used.

For the chosen device and configuration in this setup, since the input microwave signal of the NLTL is generated from a harmonic (134th) of laser repetition rate, EOS detection will mix it with the identical harmonic number (134th) (when IF = 0Hz in Fig. 3) resulting in a complex situation which down converts all NLTL harmonics to DC. As this would prevent a frequency resolved analysis, it is therefore necessary to introduce an intermediate offset (beating IF frequency) between the NLTL input signal (pump) and the laser (probe). In our case, this IF frequency is introduced by using an IQ modulator before the NLTL (see Fig. 1(c)). One should note that using an amplitude modulator (AM), as typically adopted in femtosecond optical setups is prohibitive, given the harmonics generated by AM modulation. The goal is to introduce an IF by shifting the DUT driving frequency by a constant offset with regard to the sampling frequency, while minimizing the influence of sidebands. While this signal is injected into the NLTL, due to nonlinearity, the side bands can generate spurious signals which after EOS detection can be confused with down converted harmonics of the NLTL. Hence, in the following sections we try to study the effect of this sideband in the NLTL signal in order to minimize the artifacts and errors in the measurements. In our case, by careful adjustment of the IQ modulator driving voltages and calibration, a side band reduction to −32 dBc could be achieved, as seen in Fig. 4
Fig. 4 The output signal of the IQ modulator, before injecting into the NLTL, which is up converted signal (microwave generated by laser) with an IF. The mixing of IQ modulator generates a sideband which also has one IF space from the carrier. The level of this sideband after optimization is reduced to −32dBc to minimize the artifact while mixing in the NLTL.
.

3. Quantitative consideration the effect of side bands on the NLTL signal

As an IF frequency shift can never be perfect, it is relevant to quantify the effect of finite sidebands. In the following section, we evaluate the effects induced by finite sidebands.

The operation of a NLTL can be expressed mathematically as a response function for any arbitrary x input signal with a Maclaurin power series as:
f(x)=i=0ai xi
(1)
Since the NLTL is a voltage controlled phase shifter there is no gain. Therefore an upper bound for the influence of sidebands can be derived under the assumption of a lossless NLTL. Neglecting losses (loss can be considered as a constant in the following calculation), theoretically and from energy conservation theory, the amount of input power delivered to the output is equal but with different distribution over infinite number of harmonics, therefore:
pout=pin pf(x)=px
(2)
For a single carrier input:
{x=Acos(ωct)pf(x)=px =pc=12|A|2 
(3)
and for a carrier plus a single sideband (C + SSB) input:
Acos(ωct)x +mcos[(ωm+ωc)t]Δx
(4)
while x and Δx are carrier and side band respectively, the input power spectrum is:
px+Δx=px+pΔx=12|A|2+12|m|2
(5)
Provided that modulation index:Δxx =mA1,and from Eq. (1) and (4) the NLTL output for C + SSB can be approximated by:
f(x+Δx)= i=0ai (x+Δx)i=i=0ai xi(1+Δxx)ii=0aixi(1+iΔxx)=i=0aixi+i=1ai.i.Δx.xi1=f(x)+(i=1ai.i.Δx.xi1)Δf(x)
(6)
It is notable to observe the separation of influences asf(x) only reflects the harmonics of the carrier with a regular spectrum at c, 2ωc,c, …), whileΔf(x) represents the influence of sidebands. Substituting Eq. (4) in Eq. (6) yields:
Δf(x)=mcos[(ωm+ωc)t]i=1ai.i.[Acos(ωct)]i
(7)
Thus, the spectrum of Δf(x) contains c ± ωm, 2ωc ± ωm,c ± ωm,…) frequencies, which means that only the first sideband of each carrier (harmonic) needs to be taken into account while the rest can be suppressed.

In terms of the output power spectrum from (6):
pf(x+Δx)=pf(x)+pΔf(x)
(8)
Since
pout=pin  pf(x+Δx)=px+Δx=px+pΔx
(9)
From Eq. (2), Eq. (3) and Eq. (5) it follows:
pΔf(x) =pΔx=12|m|2
(10)
These results demonstrate that for a small modulation index, the power of the side bands at the output of the NLTL equals to the input side band power and whenever the power of the sidebands is small in comparison to the carrier the effect of sidebands at the NLTL output is negligible.

4. Results

In our experiments with manual optimization (try and error) of the IQ modulator parameters such as IF (I,Q) levels and phase as well as DC offset, a C + SSB signal with RF side band level ofSBLdBc=20log(mA)=32dBc (see Fig. 4), or equally mA 0.025is achieved . This SBL fulfills the above mathematical assumption and allows neglecting sideband effects in the NLTL down converted electrooptic signals.

Using LM-LS method, jitter at IF = 10 kHz is measured at ± 3 Hz (drift of 3 × 10−4) and strongly stabilized down to a few mHz by increasing the IF to 50 kHz (drift on the order of 10−7). This stability enhancement is a consequence of a reduction of laser phase noise which decreases with increasing offset frequency. In order to emphasize on jitter reduction, comparative results of measurements (only at a slightly different IFs) on a 65 nm CMOS NLTL are shown in Fig. 5
Fig. 5 Measured signal of the NLTL output with LM-LS setup and using lock-in amplifier up to 30th harmonic (300 GHz). The injected signal of the NLTL is a microwave signal generated by laser at 10 GHz. Due decreasing jitter, we are able to measure higher harmonics with higher signal to noise ratio. The system sensitivity is restricted by shot noise level which is measured and calculated in [7].
. It can be seen that by increasing the harmonic number, the measurement with LM-LS achieves a significant gain and deviates from the measurements with LM-MS due to larger signal to noise ratio. In LM-MS method, the larger jitter at higher harmonic numbers limits the bandwidth of the measurements to the 14th harmonic (140 GHz) and beyond hits the noise floor (see Fig. 5). With LM-LS technique we are able to increase this measurement bandwidth up to 300 GHz (30th harmonic).

5. Conclusion

A Laser Master-Laser Slave configuration was introduced as a new, robust and economic solution to significantly reduce jitter in electrooptic sampling systems. According to comparative experimental results which are performed on a NLTL, It has been proven that the method has significant advantages over previously reported alternative synchronization schemes. Based on the envisaged device under test specifications this method can flexibly be designed and adapted for other EOS applications.

Acknowledgment:

We gratefully appreciate L. Tripodi (Philips Research Eindhoven) and A. Rydberg (Uppsala University) for useful discussions and the provision of the silicon based ultrafast NLTL devices [8

8. L. Tripodi, M. Matters, D. Van Goor, X. Hu, and A. Rydberg, “Extremely Wideband CMOS Circuits for Future THz Applications,” Analog Circuit Design (Springer, 2012), pp. 237–255.

]. The work leading to these results was supported with funding by the European Community's Seventh Framework Programme under grant agreement no: FP7-224189, ULTRA project [9].

References and links

1.

J. A. Valdmanis, G. A. Mourou, and C. W. Gabel, “Picosecond electro-optic sampling system,” Appl. Phys. Lett. 41(3), 211–212 (1982).

2.

H. -M. Heiliger, M. Nagel, H. G. Roskos, and H. Kurz,” Thin-film microstrip lines for mm and sub-mm-wave on-chip interconnects,” IEEE MTT-S Int. Microwave Symp. Dig.421–424 (1997).

3.

C. Kübler, R. Huber, S. Tübel, and A. Leitenstorfer, “Ultrabroadband detecetion of multi-terahertz field transients with GaSe electro-optic sensors: approaching the near infrared,” Appl. Phys. Lett. 85, 3360–3362 (2004).

4.

K. S. Giboney, S. T. Allen, M. J. W. Rodwell, and J. E. Bowers, “Picoseconds measurements by free-running electro-optic sampling,” IEEE Photon. Technol. Lett. 6(11), 1353–1355 (1994). [CrossRef]

5.

T. Pfeifer, H. M. Heiliger, T. Löffler, C. Ohlhoff, C. Meyer, G. Lüpke, H. G. Roskos, and H. Kurz, “Optoelectronic on-chip characterization of ultrafast electric devices: measurements techniques and applications,” IEEE J. Sel. Top. Quantum Electron. 2(3), 586–604 (1996). [CrossRef]

6.

K. Yang, L. P. B. Katehi, and J. F. Whitaker, “Electric field mapping system using an opticl-fiber-based electrooptic probe,” IEEE Microw. Wirel. Compon. Lett. 11(4), 164–166 (2001). [CrossRef]

7.

M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett. 54(8), 1858–1862 (2012).

8.

L. Tripodi, M. Matters, D. Van Goor, X. Hu, and A. Rydberg, “Extremely Wideband CMOS Circuits for Future THz Applications,” Analog Circuit Design (Springer, 2012), pp. 237–255.

9.

www.ultra-project.eu

OCIS Codes
(230.2090) Optical devices : Electro-optical devices
(320.7080) Ultrafast optics : Ultrafast devices

ToC Category:
Ultrafast Optics

History
Original Manuscript: October 24, 2012
Revised Manuscript: January 11, 2013
Manuscript Accepted: January 11, 2013
Published: February 13, 2013

Citation
M. Jamshidifar and P. Haring Bolívar, "Diminishing relative jitter in electrooptic sampling of active mm-wave and THz circuits," Opt. Express 21, 4396-4404 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-4-4396


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References

  1. J. A. Valdmanis, G. A. Mourou, and C. W. Gabel, “Picosecond electro-optic sampling system,” Appl. Phys. Lett.41(3), 211–212 (1982).
  2. H. -M. Heiliger, M. Nagel, H. G. Roskos, and H. Kurz,” Thin-film microstrip lines for mm and sub-mm-wave on-chip interconnects,” IEEE MTT-S Int. Microwave Symp. Dig.421–424 (1997).
  3. C. Kübler, R. Huber, S. Tübel, and A. Leitenstorfer, “Ultrabroadband detecetion of multi-terahertz field transients with GaSe electro-optic sensors: approaching the near infrared,” Appl. Phys. Lett.85, 3360–3362 (2004).
  4. K. S. Giboney, S. T. Allen, M. J. W. Rodwell, and J. E. Bowers, “Picoseconds measurements by free-running electro-optic sampling,” IEEE Photon. Technol. Lett.6(11), 1353–1355 (1994). [CrossRef]
  5. T. Pfeifer, H. M. Heiliger, T. Löffler, C. Ohlhoff, C. Meyer, G. Lüpke, H. G. Roskos, and H. Kurz, “Optoelectronic on-chip characterization of ultrafast electric devices: measurements techniques and applications,” IEEE J. Sel. Top. Quantum Electron.2(3), 586–604 (1996). [CrossRef]
  6. K. Yang, L. P. B. Katehi, and J. F. Whitaker, “Electric field mapping system using an opticl-fiber-based electrooptic probe,” IEEE Microw. Wirel. Compon. Lett.11(4), 164–166 (2001). [CrossRef]
  7. M. Jamshidifar, G. Spickermann, H. Schäfer, and P. H. Bolivar, “200-Ghz bandwidth on wafer characterization of CMOS nonlinear transmission line using electro-optic sampling,” Microw. Opt. Technol. Lett.54(8), 1858–1862 (2012).
  8. L. Tripodi, M. Matters, D. Van Goor, X. Hu, and A. Rydberg, “Extremely Wideband CMOS Circuits for Future THz Applications,” Analog Circuit Design (Springer, 2012), pp. 237–255.
  9. www.ultra-project.eu

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