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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 13 — Jun. 18, 2012
  • pp: 14683–14697
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Dynamically induced nonlinearity in a resonant-cavity interferometric intensity modulator

Janusz Murakowski, Garrett J. Schneider, and Dennis W. Prather  »View Author Affiliations


Optics Express, Vol. 20, Issue 13, pp. 14683-14697 (2012)
http://dx.doi.org/10.1364/OE.20.014683


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Abstract

The frequency dependence of the spur-free dynamic range (SFDR) in a modulator based on an injection-locked laser is analyzed. It is shown that as the modulation frequency approaches half of the locking range, the SFDR of the modulator approaches that of a standard Mach-Zehnder configuration. At low frequencies, the SFDR degrades by 2 dB for every octave of frequency increase.

© 2012 OSA

1. Introduction

Clearly, such a configuration appears attractive as it potentially allows considerable improvement of linearity with only minimal increase of system complexity. However, the approximation leading to the arcsine phase dependence on the applied signal is fully justified only for static, i.e. DC, conditions. As the frequency of the modulating signal approaches half of the locking range of the injected laser, the approximation is expected to lose its validity, and the linearity of the modulator response to be compromised. In this paper, we present a quantitative analysis of the deviation from linearity as the frequency of the modulating signal is increased from DC.

The article is organized as follows. In Sec. 2, a method of solving Adler’s equation using Fourier series expansion is described and then applied to finding the frequency response of the RCII modulator to single-tone input at both the fundamental (Sec. 2.1) and third harmonic (Sec. 2.2) frequencies. Section 2.3 extends the method to calculate the third-order intermodulation spur amplitudes when two-tone input is used. Throughout Sec. 2, we use a Fourier series expansion of the sine of trigonometric series, which is proven in Appendix A.3. The results of Sec. 2 are used in Sec. 3 to compare the SFDR of the RCII modulator to the SFDR of the Mach-Zehnder modulator, which is calculated in Appendix A.2. Discussion of the results is presented in Sec. 4, which concludes the article.

2. Fourier-series solution to Adler’s equation

The starting point for the investigations is Adler’s equation in the form presented in [7

7. A. E. Siegman, Lasers (University Science Books, 1986), pp. xxii, 1283 p.

]
dϕdt=Δω0ωmsinϕ,
(1)
where ϕ is the phase between the injected signal and the field oscillating in the cavity, Δω0 is the frequency difference between the injected signal and the free oscillations of the cavity, and ωm is half of the locking range. At DC, i.e. time-invariant deviation between the free-running resonant frequency of the injected laser and the frequency of the injected signal, the time derivative of the phase on the left-hand side of Eq. (1) vanishes, and the phase becomes an arcsine function of the frequency offset
ϕ=arcsin(Δω0ωm),
(2)
which is the basis for the linearization scheme proposed in [5

5. N. Hoghooghi, I. Ozdur, M. Akbulut, J. Davila-Rodriguez, and P. J. Delfyett, “Resonant cavity linear interferometric intensity modulator,” Opt. Lett. 35(8), 1218–1220 (2010). [CrossRef] [PubMed]

].

To study the behavior of the injected laser at modulation frequency other than zero, Eq. (1) is first simplified by redefining the frequency and time units so that ωm=1. We divide Eq. (1) by to obtain
1ωmdϕdt=Δω0ωmsinϕdϕdt˜=Δω˜0sinϕ,
where the decorated time and frequency symbols are defined as t˜=ωmt and Δω˜0=Δω0/ωm, respectively. To minimize clutter in the equations that follow, the tilde will be omitted with the understanding that the time and frequency variables are all rescaled such that ωm=1.

Modulation of the effective cavity of the injected laser is equivalent to the modulation of the offset Δω0 between the injected and the free-running frequencies. Thus, for a monochromatic, or single-tone, signal input the differential equation becomes
dϕdt+sinϕ=Ωmsin(Ωt),
(3)
where Ω is the frequency of the modulating RF signal, and Ωm is its amplitude expressed as the maximum deviation of the free-running oscillation frequency from that of the injected light.

Equation (3) has no known closed-form solutions. Below, it is solved approximately, where the approximation specifically targets the characterization of third-order nonlinearities. This means that the transient response of the system is of no interest here and therefore is omitted. In other words, it is assumed that ϕ is a periodic function of t with the period equal to the period of the driving signal, i.e.
ϕ(t+2π/Ω)=ϕ(t).
(4)
This periodicity allows the expansion of ϕ in a Fourier series

ϕ(t)=n=0ansin(nΩt+φn).
(5)

It is expected that the third-order nonlinearity will be dominant. Therefore, only terms up to n=3 in the expansion (5) above need to be considered. However, the technique developed here is more general as it allows terminating the expansion after an arbitrary number of terms, which makes it applicable in other types of modulators.

With the help of Theorem (46) (found in the Appendix) and the Fourier expansion (5), Eq. (3) can be written as
Ωnnancos(nΩt+φn)+knJkn(an)sin[nkn(nΩt+φn)]=Ωmsin(Ωt),
(6)
where Jkn(an) is a Bessel function of order kn with argument an (see Appendix for a complete explanation of the notation). Due to the linear independence of terms corresponding to different frequencies, Eq. (6) separates into a system of infinitely many equations, each containing terms corresponding to a single frequency, an integer multiple of Ω.

2.1 First-order approximation

Initially, only the fundamental response at is considered. In this case, only n=1 and k1=±1 need to be considered since all other combinations involve higher order terms, higher frequencies, or constants. Therefore, an=0 for n>1, and Eq. (6) becomes
Ωa1cos(Ωt+φ1)+J1(a1)sin(Ωt+φ1)+J1(a1)sin[(Ωt+φ1)]=Ωmsin(Ωt).
(7)
Since J1(z)=J1(z) and sin(z)=sin(z), Eq. (7) can be simplified to
Ωa1cos(Ωt+φ1)+2J1(a1)sin(Ωt+φ1)=Ωmsin(Ωt).
(8)
Equation (8) must hold at all values of t. Inserting into Eq. (8) values of t such that Ωt+φ1=0 and Ωt+φ1=π/2 shows that the amplitude a1 and phase φ1 must satisfy the following system of equations
Ωa1=Ωmsin(φ1)2J1(a1)=Ωmcos(φ1).
(9)
Equations (9) are a system of nonlinear equations that can be solved for parameters a1 and φ1 in terms of the frequency and amplitude of the modulating signal, Ω and Ωm, respectively. The result is the linear response of the phase ϕ to the modulation. In other words, ϕ is the following function of time:
ϕ(t)=a1sin(Ωt+φ1),
(10)
with a1 and φ1 obtained from Eqs. (9) as functions of Ω and Ωm.

As mentioned in the Introduction, to detect the phase modulation ϕ in a physical system, the modulated signal is interfered with a reference to produce amplitude modulation at a photo-detector. Mathematically, this procedure amounts to calculating the sine of phase ϕ (assuming quatrature bias). Using Theorem (46) we find
sin[ϕ(t)]=sin[a1sin(Ωt+φ1)]=k=Jk(a1)sin[k(Ωt+φ1)].
(11)
Since the approximation (10) only accounts for the first-order response, in the series of Eq. (11) only the terms with |k|1 are meaningful:
sin[ϕ(t)]k=11Jk(a1)sin[k(Ωt+φ1)]=2J1(a1)sin(Ωt+φ1)A1sin(Ωt+φ1),
(12)
where A1 is defined in the last equality as the amplitude of the linear response of the modulator.

For small modulation amplitude, Ωm1, the system of Eqs. (9) can be simplified. In this case, the first of Eqs. (9) leads to a11, which means that in the second of Eqs. (9) only the linear term in the Taylor expansion of the Bessel function, J1(a1)=a1/2a13/16+o(a13), is significant. As a result, Eqs. (9) become
Ωa1=Ωmsin(φ1)a1=Ωmcos(φ1),
(13)
which, by using trigonometric identity sin2(φ1)+cos2(φ1)=1, can be solved to obtain the following expression for coefficient a1 as a function of modulation frequency
a1=Ωm1+Ω2.
(14)
The linear response amplitude A1 defined in Eq. (12) simplifies for small input amplitude Ωm to
A1=2J(a1)a1=Ωm1+Ω2.
(15)
Expression (15) for the frequency dependence of the modulator response is identical to a low-pass filter. Figure 2
Fig. 2 Magnitude of the frequency response of the RCII modulator in the first-order approximation.
shows the frequency response of the modulator normalized to 0 dB at DC. The 3-dB drop occurs at the modulation frequency equal to half of the locking range, i.e. at Ω=ωm.

2.2 Third-order approximation

Next, the contributions to the third harmonic of the modulation signal are examined. In this case, in Eq. (6) we only need to consider n{1,3}, k1{0,±1,±3}, k3{0,±1}. The following combinations of parameters contribute to the 3Ω term:
k1=±3,k3=0;andk1=0,k3=±1.
Note that cases k1=2,k3=1 and k1=2,k3=1 are omitted as they give rise to higher-order (5-th) amplitude terms. Since in this case a1 and a3 can take nonzero values, the equations for amplitude and phase at frequency Ω are modified from Eq. (9). The terms that contribute to frequency Ω correspond to
k1=±1,k3=0.
With the series truncated to the terms listed above, Eq. (6) becomes
Ωa1cos(Ωt+φ1)+3Ωa3cos(3Ωt+φ3)+2J1(a1)J0(a3)sin(Ωt+φ1)+2J3(a1)J0(a3)sin(3Ωt+3φ1)+2J0(a1)J1(a3)sin(3Ωt+φ3)=Ωmsin(Ωt),
(16)
where the anti-symmetry of the odd-order Bessel functions has been used (compare Eqs. (7) and (8)). The linear independence of terms corresponding to different frequencies leads to the separation of Eq. (16) into two independent equations
Ωa1cos(Ωt+φ1)+2J1(a1)J0(a3)sin(Ωt+φ1)=Ωmsin(Ωt)3Ωa3cos(3Ωt+φ3)+2J3(a1)J0(a3)sin(3Ωt+3φ1)+2J0(a1)J1(a3)sin(3Ωt+φ3).=0
(17)
Note that the first of Eqs. (17) differs from Eq. (8) by the presence of J0(a3) in the second term. This is because in the first-order analysis above, a3 was assumed to be zero, which is not the case here. Since Eqs. (17) have to hold at any time t, the following relations between the amplitudes and phases must be satisfied
a1Ω=Ωmsin(φ1)2J1(a1)J0(a3)=Ωmcos(φ1)3Ωa3+2J3(a1)J0(a3)sin(3φ1φ3)=02J3(a1)J0(a3)cos(3φ1φ3)+2J0(a1)J1(a3)=0.
(18)
Equations (18) are a system of four nonlinear equations that allows the calculation of amplitudes a1 and a3, as well as phases φ1 and φ3 in terms of the modulation frequency Ω and amplitude Ωm. Using this solution, ϕ(t) can be expressed up to the third order as

ϕ(t)=a1sin(Ωt+φ1)+a3sin(3Ωt+φ3).
(19)

To detect the phase modulation ϕ in a physical system, the modulated signal is interfered with a reference to produce amplitude modulation at a photo-detector. Mathematically, this procedure amounts to calculating the sine of phase ϕ (assuming quadrature bias). By rearranging Eq. (3), we find that
sinϕ=Ωmsin(Ωt)dϕdt.
(20)
Differentiating expression (19) and substituting the result into the right side of Eq. (20) yields
sinϕ=Ωmsin(Ωt)Ωa1cos(Ωt+φ1)3Ωa3cos(3Ωt+φ3).
(21)
Expression (21) shows that the spectral content of the signal detected at the output of the RCII modulator includes the fundamental frequency Ω of the input signal, and its third harmonic 3Ω. Such signal has a general form
sin[ϕ(t)]A1sin(Ωt+ϕ1)+A3sin(3Ωt+ϕ3),
(22)
where A1,ϕ1,A3,ϕ3 are defined to be the amplitudes and phases of the modulator response at the fundamental and third harmonic, respectively. The amplitude A3 of the third harmonic is found immediately by inspection of Eq. (21), and is equal to (up to a sign)
A3=3Ωa3,
(23)
whereas the amplitude of the fundamental tone is found by evaluating the first two terms in Eq. (21) at values of t such that Ωt=0 and Ωt=π/2 and combining the results in quadrature
A1={(Ωa1cosφ1)2+[ΩmΩa1cos(π2+φ1)]2}1/2=[(Ωa1cosφ1)2+(Ωm+Ωa1sinφ1)2]1/2=[(Ωa1)2+Ωm2+2Ωa1Ωmsinφ1]1/2=[(Ωa1)2+Ωm2]1/2,
(24)
where the last equality is obtained by using the first of Eqs. (18). Formulas (24) and (23), where a1 and a3 are solutions to the system of Eqs. (18), provide a basis for calculating the frequency dependence of third-order spurs in the modulator.

The calculation of the spur-free dynamic range relies on the extrapolation of small-signal system response. In this case, both a1 and a3 are small, and it is sufficient to keep only the lowest-order terms in these parameters in Eqs. (18). Upon Taylor expansion of the Bessel functions, keeping only terms up to third order in a1 and a3, Eqs. (18) become
a1Ω=Ωmsin(φ1)a1=Ωmcos(φ1)3Ωa3+a1324sin(3φ1φ3)=0a1324cos(3φ1φ3)+a3=0.
(25)
By applying the trigonometric identity sin2α+cos2α=1 on the first and second pairs of Eqs. (25), the phases are eliminated, and the following relations between amplitudes a1 and a3 are obtained
a1=Ωm1+Ω2a3=a13241+(3Ω)2.
(26)
Combining Eqs. (24), (23), and (26) yields the small signal approximation for the fundamental and third-harmonic response amplitudes
A1=Ωm1+Ω2,A3=ΩA1381+(3Ω)2=ΩΩm38(1+Ω2)3/21+(3Ω)2.
(27)
Expressions (27) show the fundamental and third-harmonic response of the modulator to a single-tone modulation at frequency Ω and amplitude Ωm. The fundamental-frequency response of Eq. (27) is identical to that calculated when only linear terms were considered in the analysis, Eq. (15), as expected. The third-harmonic content is frequency dependent, and goes to zero when the modulation frequency Ω goes to zero, i.e. the system becomes linear, up to third order calculated here, in the DC limit, as anticipated from the DC limit of the Adler’s equation discussed in the introduction.

2.3 Third-order intermodulation

Substituting the series expansion (29) into Adler’s Eq. (28) yields, with the help of Theorem (46), the following equation
nnΩancos(nΩt+φn)+knJkn(an)sin[nkn(nΩt+φn)]=Ωm[sin(Ω1t)+sin(Ω2t)],
(30)
which separates into a system of infinitely many equations, each containing terms corresponding to a single frequency—due to the linear independence of terms corresponding to different frequencies.

The linear response is obtained by keeping the lowest-order terms in Eq. (30), which separates then into two equations—one for each of the input frequencies
Ω1a(1,0)cos(Ω1t+φ(1,0))+2J1(a(1,0))sin(Ω1t+φ(1,0))=Ωmsin(Ω1t),Ω2a(0,1)cos(Ω2t+φ(0,1))+2J1(a(0,1))sin(Ω2t+φ(0,1))=Ωmsin(Ω2t).
(31)
Each of Eqs. (31) is identical with Eq. (8), and yields the same linear-response solution as given in Eq. (14)
a(1,0)=Ωm1+Ω12,a(0,1)=Ωm1+Ω22.
(32)
To calculate the amplitudes of third-order intermodulation spurs, the terms contributing to frequencies 2Ω1Ω2 and 2Ω2Ω1 in the series Eq. (30) must be identified. It can be shown that at frequency 2Ω1Ω2, corresponding to one of the third-order spurs, Eq. (30) yields
(2Ω1Ω2)a(2,1)cos[(2Ω1Ω2)t+φ(2,1)]+2J1(a(2,1))sin[(2Ω1Ω2)t+φ(2,1)]2J2(a(1,0))J1(a(0,1))sin[(2Ω1Ω2)t+2φ(1,0)φ(0,1)]=0.
(33)
The first term in Eq. (33) corresponds to the only term in the first series of Eq. (30) with frequency 2Ω1Ω2, whereas the last two terms of Eq. (33) are the lowest-order terms in the second series of Eq. (30) with the same frequency. Note the explicit absence of the input amplitude Ωm in Eq. (33). The dependence of the intermodulation amplitude a(2,1) on the input amplitude Ωm is indirect—via linear-response amplitudes a(1,0) and a(0,1) found in Eqs. (32). By using techniques similar to those used in Sec. 2.2 when calculating the amplitude of the third harmonic, Eq. (33) can be solved for coefficient a(2,1) in the dual Fourier series expansion (29)
a(2,1)=a(1,0)2a(0,1)81+(2Ω1Ω2)2,
(34)
and the amplitude of the intermodulation term with frequency 2Ω1Ω2 at the output of the RCII modulator obtained from sinϕ is found to be
A(2,1)=(2Ω1Ω2)a(1,0)2a(0,1)81+(2Ω1Ω2)2=Ωm38(1+Ω12)1+Ω221+(2Ω1Ω2)2.
(35)
Similarly, the output amplitude at intermodulation frequency 2Ω2Ω1 is found
A(1,2)=(2Ω2Ω1)a(0,1)2a(1,0)81+(2Ω2Ω1)2=Ωm38(1+Ω22)1+Ω121+(2Ω2Ω1)2.
(36)
For frequencies Ω1 and Ω2 such that the difference between them is much smaller than either of these frequencies, |Ω1Ω2|Ω1 and |Ω1Ω2|Ω2, i.e. Ω1Ω2=Ω, expressions for the spur amplitudes (35) and (36) simplify to
Aspur=Ωa1381+Ω2=ΩΩm38(1+Ω2)2,
(37)
where a1 is the linear coefficient of the Fourier-expansion solution, Eqs. (15), (26), or (32), and at the same time is equal to the (linear) output amplitude of the modulator at the fundamental frequency, Eqs. (15) or (27). Note that the expression for the intermodulation amplitude Eq. (37) is similar in structure to the third-harmonic amplitude Eq. (27). This should come as no surprise since both are the result of the same underlying nonlinear phenomena of the RCII modulator.

3. Spur-free dynamic range of RCII modulator

Given the definition of SFDR provided in Appendix A.1, to make the comparison of SFDR in the RCII modulator with the same in Mach-Zehnder modulator, only the output coordinates of the third-order intercept points (OIP3) are required for the two modulator configurations. The OIP3 for the Mach-Zehnder is calculated in the Appendix, Eq. (45). In this section, the OIP3 for the RCII is obtained. It is easily found by noting that—by the definition of intercept point—when the modulation amplitude Ωm is equal to the input coordinate of the intercept point (IIP3(RCII)), the output amplitude of the linear response A1 given, e.g., in Eq. (27), is equal to the amplitude of the spur Aspur found in Eq. (37)
A1=Ωm1+Ω2=ΩΩm38(1+Ω2)2=Aspur.
(38)
Solving Eq. (38) for Ωm yields the input coordinate of the intercept point
IIP3(RCII)=8(1+Ω2)3/2Ω.
(39)
The output coordinate of the intercept point OIP3(RCII) is obtained from IIP3(RCII) by multiplying the latter with the (linear) gain of the system, which is simply the ratio of the output amplitude at the fundamental frequency over the amplitude of the modulating signal A1/Ωm=(1+Ω2)1/2. The result is
OIP3(RCII)=22(1+Ω2)1/4.
(40)
The ratio of this value to the output intercept coordinate corresponding to the Mach-Zehnder configuration found in Eq. (45) is the amount of linearity improvement of RCII modulator over the Mach-Zehnder modulator. Converted to dB, the ratio becomes a difference
SFDR(RCII)SFDR(MZ)=2320log10(1+Ω2)1/4[dB-Hz2/3],
(41)
where the factor of 2/3 accounts for the same factor present when calculating SFDR, see Fig. 4 in Appendix A.1. Note that calculating this difference renders the exact position of the noise floor irrelevant to the comparison of the SFDR-s of two modulators. Expression (41) for the SFDR improvement in RCII modulator over MZ modulator is plotted in Fig. 3
Fig. 3 Improvement of the SFDR in the RCII modulator over a Mach-Zehnder (MZ) modulator as a function of modulation frequency.
.

4. Discussion

Based on Eqs. (15) and (41), and Figs. 2 and 3, the following observations can be made regarding the performance of the resonant-cavity interferometric intensity modulator (RCII) as compared to the conventional Mach-Zehnder (MZ) configuration.

  • 1. Modulation efficiency of the RCII modulator falls as the modulation frequency is increased; the 3-dB point is reached at Ω=ωm, i.e. when the modulation frequency is equal to half of the locking range.
  • 2. SFDR of the RCII modulator approaches that of the MZ modulator when the modulation frequency approaches half of the locking range, i.e. when Ωωm.
  • 3. With every octave of modulation frequency increase, the SFDR drops by ~2 dB as long as the modulator is operated at frequencies below about 25% of the locking range. For higher frequencies, the SFDR of the RCII modulator asymptotically converges to the SFDR of the MZ modulator.
  • 4. If we consider a practical example where the injected laser is set to operate with 10-GHz locking range, then operating the modulator at frequencies up to 150 MHz (3% of half locking range) would produce a relatively modest 10 dB improvement of SFDR over the Mach-Zehnder modulator. This result seems to stand in direct contradiction with the conclusions of [5

    5. N. Hoghooghi, I. Ozdur, M. Akbulut, J. Davila-Rodriguez, and P. J. Delfyett, “Resonant cavity linear interferometric intensity modulator,” Opt. Lett. 35(8), 1218–1220 (2010). [CrossRef] [PubMed]

    ] where hundreds of dB in SFDR improvement in the RCII over the Mach-Zehnder have been hypothesized. In fact, no such contradiction exists since the results of [5

    5. N. Hoghooghi, I. Ozdur, M. Akbulut, J. Davila-Rodriguez, and P. J. Delfyett, “Resonant cavity linear interferometric intensity modulator,” Opt. Lett. 35(8), 1218–1220 (2010). [CrossRef] [PubMed]

    ] are strictly valid only for the modulation frequency equal to zero, whereas the analysis presented here extends that range to arbitrary modulation frequencies. Using Eq. (41), it can be calculated that in order to obtain a 100-dB improvement of SFDR in the RCII modulator over a Mach-Zehnder, the modulation frequency must not exceed 1.0×1015ωm, which in the example considered here corresponds to 5 μHz.

Intuitively, the frequency limitation of the RCII modulator can be thought of as follows. As the modulation frequency approaches the edge of the locking range, it is expected that the oscillator will be hard-pressed to follow the modulating signal. Therefore, the phase-modulation response will be diminished—hence the low-pass-filter behavior and 3-dB drop seen in Fig. 2 at Ω=ωm. At the same time, since the locking phenomenon plays a diminished role at these modulation frequencies, the improvement in linearity over a conventional MZ modulator is expected to be erased as well. At low modulating frequencies, the 2-dB/octave improvement in SFDR as the frequency is reduced is the result of the 2/3 factor that accompanies the calculation of the SFDR.

To achieve at least 10 dB of improvement in SFDR over Mach-Zehnder, the RCII modulator would have to operate at frequencies below 3% of the half-locking range. If we set the operating frequency at 10 GHz, a reasonable bandwidth for practical applications, it means the half-locking range needs to be of the order of 300 GHz. Since the half-locking range is determined by the locked oscillator Q and the ratio of the injected power I1 to the output power I0 according to [7

7. A. E. Siegman, Lasers (University Science Books, 1986), pp. xxii, 1283 p.

],
ωm=ω0QI1I0,
(42)
such wide locking range can be achieved by either reducing the Q of the laser cavity or by increasing the injection power relative to the output power of the injected laser. The injection ratio is limited by the emergence of nonlinearities and instabilities, as analyzed in Ref [8

8. N. Hoghooghi and P. J. Delfyett, “Theoretical and experimental study of a semiconductor resonant cavity linear interferometric intensity modulator,” J. Lightwave Technol. 29(22), 3421–3427 (2011). [CrossRef]

]. On the other hand, reducing the Q of the laser cavity requires higher pump currents to achieve lasing, but may be a preferable avenue. In this case, reducing the cavity size would be beneficial. Therefore, it is expected that VCSELs would be preferable over DFB lasers in this regard. Even better, photonic-crystal lasers or plasmonic lasers [9

9. R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]

] occupying a fraction of λ3 could outperform other types of lasers in RCII modulators.

Appendix

A.1 Spur-free dynamic range (SFDR)

SFDR is defined as a difference (on a dB scale) between the minimum detectable input signal level, and the input level which will produce distortion products (spurs) equal to the minimum detectable signal referred to the input of the system [3

3. “Spurious-free dynamic range,” Wikipedia, The Free Encyclopedia,http://en.wikipedia.org/wiki/Spurious-free_dynamic_range, (accessed April 9, 2012).

, 4

4. W. F. Egan, Practical RF system design (IEEE Press; Wiley-Interscience, 2003), pp. xxv, 386 p.

]. For a system limited by third-order nonlinearity, such as the modulator analyzed in this article, the situation is illustrated in Fig. 4
Fig. 4 Obtaining SFDR from the coordinates of the third-order spur intercept and the noise floor. SFDR is equal to 2/3 of the separation of the OIP3 from the noise floor.
. The linear response of the system is shown as a green line with a slope of 1 (on dB scale). The minimum detectable signal is determined by the noise floor (for a pre-defined finite signal bandwidth), and the system gain that controls the position (but not the slope) of the green line on the graph. The input coordinate of the intersection of the linear response (green line) with the noise floor (horizontal blue line) is the minimum detectable signal.

The red line represents the third-order spurs. On the dB scale, the spur line has a slope of 3. The input coordinate of the intersection of the spur (red) line with the noise floor is the (input) signal level that produces spurs equal to the minimum detectable signal. According to the definition provided above, the SFDR is the difference between the input coordinates of the intersection of the red line with the blue line, and the input coordinate of the intersection of the green line with the blue line, as indicated in Fig. 4 by the horizontal interval labeled SFDR. Note that since the slope of the linear-response line is 1 in this graph, the SFDR can also be defined with respect to output signal levels as indicated by the vertical interval labeled SFDR.

Because the linear response line has a slope 1, and the spur line has a slope 3, the two lines intersect. Furthermore, if any useful signal is to be recovered, the intersection has to occur above the noise floor. Define IIP3 as the input coordinate of the intercept point between the linear-response line and the spur line. Similarly, define OIP3 as the output coordinate of the intercept point between the linear-response line and the spur line. Since the slope of the green line is 1 and the slope of the red line is 3, it is easy to ascertain using elementary geometry that the SFDR, measured using the vertical interval, is 2/3 of the difference between the OIP3 and the noise floor. This is the definition of the SFDR that is used here.

A.2 Third-order spurs in Mach-Zehnder modulator

As an illustration of the method of finding the intercept with the third-order spur discussed at the beginning of Sec. 3, consider a Mach-Zehnder (MZ) modulator. If the modulator is biased in quadrature, the transfer function is
f(x)=sin(x).
(43)
To obtain intermodulation, a superposition of two sine-waves with the same amplitude, but different frequencies is used for the argument of the transfer function
f(msin(Ω1t)+msin(Ω2t))=sin[msin(Ω1t)+msin(Ω2t)].
The series expansion of the sine function of a trigonometric series is carried out according to the Theorem of Appendix A.3, Eq. (46). The series is then truncated at lowest-order terms in the modulation amplitude m
f(x)=sin[msin(Ω1t)+msin(Ω2t)]=k1,k2=Jk1(m)Jk2(m)sin[(k1Ω1+k2Ω2)t]2J1(m)J0(m)sin(Ω1t)+2J0(m)J1(m)sin(Ω2t)2J2(m)J1(m)sin[(2Ω1Ω2)t]2J1(m)J2(m)sin[(2Ω2Ω1)t]2J1(m)J0(m)[sin(Ω1t)+sin(Ω2t)]2J2(m)J1(m){sin[(2Ω1Ω2)t]+sin[(2Ω2Ω1)t]}(m38m3)[sin(Ω1t)+sin(Ω2t)]18m3{sin[(2Ω1Ω2)t]+sin[(2Ω2Ω1)t]},
(44)
where the last equality is obtained by Taylor expansion of the Bessel functions and retaining terms up to third order in the modulation amplitude m. Equating the coefficients of the linear response and the third-order intermodulation amplitude in the last of Eqs. (44),
m=18m3,
immediately yields the third-order spur output intercept coordinate
OIP3(MZ)=22,
(45)
which is identical to the input coordinate of the third-order spur intercept point IIP3(MZ) since the (linear) gain of the system is one, per the last of Eqs. (44).

Note that the frequency response of an ideal MZ modulator considered here is flat, i.e. the output is frequency independent, see Eq. (43). As a result, the output intercept coordinate given in Eq. (45) that characterizes the third-order spurs does not depend on the frequency of the input signal.

A.3 Fourier expansion of the sine of trigonometric series

Theorem:

sin(nansinxn)=knJkn(an)sin(kx),
(46)

where k=(k1,k2,,kn,), each kn spans all integers (kn,n), and kx=nknxn.

Proof:

Begin by rewriting the outer sine function in exponential form, and replacing the exponential of a sum with the product of the exponentials of each term,
sin(nansinxn)=12i[exp(inansinxn)exp(inansinxn)]=12i[nexp(iansinxn)nexp(iansinxn)].
Using Fourier expansion of the exponential function
exp(izsinϕ)=k=Jk(z)eikϕ,
(47)
where coefficients Jk(z) are Bessel functions of the first kind, with z=an and ϕ=±xn, the last expression can be written as
sin(nansinxn)=12i[nkn=Jkn(an)exp(iknxn)nkn=Jkn(an)exp(iknxn)].
The distributive property of multiplication over addition, and commutative and associative properties of multiplication allow the following:
sin(nansinxn)=12i[k1=k2=kn=nJkn(an)exp(iknxn)k1=k2=kn=nJkn(an)exp(iknxn)]=12i[knJkn(an)exp(iknxn)knJkn(an)exp(iknxn)]=12i{k[nJkn(an)][nexp(iknxn)]k[nJkn(an)][nexp(iknxn)]}=12i{k[nJkn(an)]exp(inknxn)k[nJkn(an)]exp(inknxn)}=k[nJkn(an)]12i(eikxeikx)=knJkn(an)sin(kx),
which completes the proof.

References and links

1.

C. H. Cox, E. I. Ackerman, G. E. Betts, and J. L. Prince, “Limits on the performance of RF-over-fiber links and their impact on device design,” IEEE Trans. Microw. Theory Tech. 54(2), 906–920 (2006). [CrossRef]

2.

D. J. F. Barros and J. M. Kahn, “Optical modulator optimization for orthogonal frequency-division multiplexing,” J. Lightwave Technol. 27(13), 2370–2378 (2009). [CrossRef]

3.

“Spurious-free dynamic range,” Wikipedia, The Free Encyclopedia,http://en.wikipedia.org/wiki/Spurious-free_dynamic_range, (accessed April 9, 2012).

4.

W. F. Egan, Practical RF system design (IEEE Press; Wiley-Interscience, 2003), pp. xxv, 386 p.

5.

N. Hoghooghi, I. Ozdur, M. Akbulut, J. Davila-Rodriguez, and P. J. Delfyett, “Resonant cavity linear interferometric intensity modulator,” Opt. Lett. 35(8), 1218–1220 (2010). [CrossRef] [PubMed]

6.

R. Adler, “A study of locking phenomena in oscillators,” Proc. IRE 34, 351–357 (1946).

7.

A. E. Siegman, Lasers (University Science Books, 1986), pp. xxii, 1283 p.

8.

N. Hoghooghi and P. J. Delfyett, “Theoretical and experimental study of a semiconductor resonant cavity linear interferometric intensity modulator,” J. Lightwave Technol. 29(22), 3421–3427 (2011). [CrossRef]

9.

R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]

OCIS Codes
(060.4080) Fiber optics and optical communications : Modulation
(060.5060) Fiber optics and optical communications : Phase modulation
(140.3520) Lasers and laser optics : Lasers, injection-locked
(230.4110) Optical devices : Modulators
(140.3518) Lasers and laser optics : Lasers, frequency modulated
(250.4110) Optoelectronics : Modulators

ToC Category:
Optical Devices

History
Original Manuscript: January 24, 2012
Revised Manuscript: April 14, 2012
Manuscript Accepted: June 4, 2012
Published: June 15, 2012

Citation
Janusz Murakowski, Garrett J. Schneider, and Dennis W. Prather, "Dynamically induced nonlinearity in a resonant-cavity interferometric intensity modulator," Opt. Express 20, 14683-14697 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-13-14683


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References

  1. C. H. Cox, E. I. Ackerman, G. E. Betts, and J. L. Prince, “Limits on the performance of RF-over-fiber links and their impact on device design,” IEEE Trans. Microw. Theory Tech.54(2), 906–920 (2006). [CrossRef]
  2. D. J. F. Barros and J. M. Kahn, “Optical modulator optimization for orthogonal frequency-division multiplexing,” J. Lightwave Technol.27(13), 2370–2378 (2009). [CrossRef]
  3. “Spurious-free dynamic range,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/wiki/Spurious-free_dynamic_range , (accessed April 9, 2012).
  4. W. F. Egan, Practical RF system design (IEEE Press; Wiley-Interscience, 2003), pp. xxv, 386 p.
  5. N. Hoghooghi, I. Ozdur, M. Akbulut, J. Davila-Rodriguez, and P. J. Delfyett, “Resonant cavity linear interferometric intensity modulator,” Opt. Lett.35(8), 1218–1220 (2010). [CrossRef] [PubMed]
  6. R. Adler, “A study of locking phenomena in oscillators,” Proc. IRE 34, 351–357 (1946).
  7. A. E. Siegman, Lasers (University Science Books, 1986), pp. xxii, 1283 p.
  8. N. Hoghooghi and P. J. Delfyett, “Theoretical and experimental study of a semiconductor resonant cavity linear interferometric intensity modulator,” J. Lightwave Technol.29(22), 3421–3427 (2011). [CrossRef]
  9. R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature461(7264), 629–632 (2009). [CrossRef] [PubMed]

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