拉曼產生之光頻梳

Raman-Generated Frequency Comb

謝智明(Zhi-Ming Hsieh)

指導教授 ： 易富國

共同指導教授 ： 孔慶昌

國立臺灣大學/理學院/物理研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.00008

摘要

在這篇論文中，我們首先描述在氫分子中使用拉曼分子同調技術所產生的七道同調光來合成週期性的脈衝光鏈，可達0.83週期且內部電場的脈衝半高寬度為0.44飛秒。拉曼方式所產生的光頻梳，是用絕熱的驅動分子振動同調。我們證明用交互相干技術，當光頻梳為相稱的頻率時，這些波形的載波包絡相位為常數值。在約百萬根的脈衝光鏈中，從第一根到最後一根光鏈，其載波包絡相位的改變差異約略估計，小於0.18個週期。然而載波包絡相位在這一發到下一發的光脈衝之間為一任意的常數值。
承隨了證明光脈衝為載波包絡相位為常數之後，我們為了解決常數之載波包絡相位在不同發的脈衝光相位為任意值的問題，為以固定脈衝光在發與發之間的相位，我們使用了單一道的高強度基本光源及其所產生的非線性二倍頻光源一起去驅動拉曼過程。我們論證了這些由拉曼過程產生光頻梳的超短週期性波形中，其載波包絡相位是可以控制的。超外插測量證明了所有光頻梳內相互光頻之間的相位是固定的。這些結果說明了在飛秒及次飛秒等級之超短光脈衝範圍裡，我們能夠完全整型合成週期性的任意光脈衝波形。
延續之前的工作，控制了由拉曼過程產生的週期性光脈衝鏈裡之載波包絡相位，我們繼續朝著我們的目標前進，亦即在可見光附近的範圍內產生一個任意形狀的光脈衝波形。在能夠控制載波包絡相位技術的基礎上，整形合成任意波形的光脈衝，在我們的實驗裡，僅僅只須再加上控制光頻裡每一道光強弱。這其餘的工作只是在如何去證明、確認這個實驗結果。有幾種方法技術可以應用在這個實驗中，非線性相干諸如四波混頻或是和頻的方法是常被提及的。在這裡，我們提出另一種方式，即線性相干量測，來測量這任意光脈衝的波形。我們使用了一根根光頻梳的整型技術去合成一亞週期，短於兩飛秒內的光脈衝波形。藉由光脈衝自身的”輔助整形”相干技術而不是其它的非線性方法，諸如四波混頻、頻譜解析光閘或是其它的方法，證實能成功的合成多種的波形。
最近，用同調控制的脈衝整型和用飛秒鎖模雷射來合成波形已經能夠同時地合成與描述超短光脈衝的特性。脈衝整型是將一道飛秒等級的光脈衝分成兩道相同光脈衝，然後其中一道光在時間上相對於另一道光做時間延遲。這種相干量測是在時間上做電子式的延遲而獲得的。實際上，光脈衝本身並不須要真的分成兩道光而且本身亦不須要做機戒式的掃描。這種方式消除了寬頻鍍膜上的困難及機械裝置上的不穩定性等問題。這種方法特別適合單週期光脈衝的發展。相干訊號顯示了和將光脈衝分成兩道光的相互相干方式做比較，在譟訊比和準確度上有相當程度的改善。輔助整型相干技術在超短光脈衝範圍內的量測上是一種很有用的量測技術，其特別適合用在數根光的光頻梳上。
在完成上述的先前工作之後，前常數值的載波包絡相位到亞週期、短於兩飛秒的任意光脈衝合成上，我們持續朝下一個目標前進，合成一亞飛秒等級的任意光脈衝波形。為了達到合成亞飛秒等級任意光脈衝波形的這個目標，我們須要使用更多道由拉曼過程所產生的光。我們加了第三道入射驅動光來增強拉曼光頻梳的高階項的能量。這能夠利用基頻光的八倍頻光來當作第三道驅動光源。我們已經獲得初步的結果，且明顯地改善了拉曼光頻梳的高階項能量(波長為344,301,267及241奈米)。

關鍵字

分子調制 ； 拉曼產生之光頻梳 ； 載波包絡相位 ； 任意波形 ； 空間光調制器

並列摘要

In this thesis, we first describe the synthesis of periodic waveforms consisting of a pulse trains that are 0.83 cycles long and having an inner electric field pulse width of 0.44 femtosecond using 7 Raman sidebands generated by molecular modulation in hydrogen gas. We generated a Raman-generated frequency comb by adiabatically driving a molecular vibrational coherence. We verify by optical cross correlation that the carrier-envelope phase is constant in these waveforms when they are synthesized from commensurate sidebands. The estimated overall shift of the carrier-envelope phase is less than 0.18 cycles from the first to the last pulse of nearly 10 6 pulses in the pulse train. However, the constant carrier envelope phases are random from shot to shot.
We follow the work of the constant carrier envelope phase and solve the problem that the constant carrier envelop phases are random. In order to fix the phase of pulses from shot to shot, we use an intense fundamental light source and its second harmonic to drive the Raman process. We demonstrate control of the carrier-envelope phase of ultrashort periodic waveforms that are synthesized from a Raman-generated optical frequency comb. Heterodyne measurements show that full interpulse phase locking of the comb components is realized. The results set the stage for the synthesis of periodic arbitrary waveforms in the femtosecond and subfemtosecond regimes with full control.
To continue our last work in controlling the CEP of Raman-generated frequency comb with a periodic optical pulse trains, we keep going on to our goals; that is generating an optical arbitrary waveform in the optical regime. Base on the controlling of the CEP, synthesized arbitrary waveform just in adding to control the amplitude of each sideband in our experiment. The remainding work is how to confirm or prove the experimental results. There are some techniques to apply in this experiment. Nonlinear correlations such as four wave mixing or sum frequency mixing are often adopted. Here we presented another method, linear correlation, to measure the arbitrary waveform. We synthesize a sub-cycle, sub-two-femtosecond optical waveforms using line-by-line shaping of the frequency comb. The successful syntheses of various waveforms are verified by shaper-assisted cross-correlation of pulses created from the waveform itself rather than nonlinear method such as four-wave mixing, frequency resolved optical gating (FROG), or others.
Recently a pulse-shaper used in coherent control and waveform synthesis of femtosecond mode-locked lasers has been employed to perform simultaneous synthesis and charaterization of ultrafast waveforms. The pulse shaper splits a femtosecond optical pulse into two equal pulses in time with a variable time delay between the two split pulses. Correlation measurements are obtained by sweeping the time delay electronically. The pulse does not need to be split mechanically and there is no mechanical scanning. This eliminates the problem of broadband coatings and mechanical instability. It is developed to characterize single-cycle pulses. The correlation signal show significantly improved signal-to-noise ratio and thus accuracy in pulse characterization when compared to cross-correlation obtained by splitting the spectral components of the pulse. The shaper-assisted correlation technique is a powerful tool in the measurements of ultrashort range, especially appropriate in the frequency comb of a few components.
After the previously works, from constant carrier envelope phase to sub-cycle and sub-two-femtosecond arbitrary optical waveform synthesized, we go forward the next goals, synthesize a sub-femtosecond arbitrary optical waveform. In order to achieve the goal of synthesizing subfemtosecond optical waveforms more components of the Raman-generated optical frequency comb must be used. We add a 3rd driving beam to enhance power in high-order components of Raman-generated frequency comb. This can be done by generating the 8-th harmonic of the fundamental frequency as the 3rd driving beam. The preliminary results are obtained and significant improvement in high-order components (344 nm, 301 nm, 267 nm and 241 nm) of Raman-generated frequency comb.

並列關鍵字

molecular modulation ； Raman-generated frequency comb ； carrier envelope phase ； shaper-assisted correlation ； arbitrary waveform ； spatial light modulator

參考文獻

1. Corkum, P., Breaking the Attosecond Barrier. Opt. Photon. News, 1995. 6(5): p. 18-22.

2. Krausz, F., et al., EXTREME NONLINEAR OPTICS: EXPOSING MATTER TO A FEW PERIODS OF LIGHT. Opt. Photon. News, 1998. 9(7): p. 46-51.

3. Brabec, T. and F. Krausz, Intense few-cycle laser fields: Frontiers of nonlinear optics. Reviews of Modern Physics, 2000. 72(Copyright (C) 2009 The American Physical Society): p. 545.

4. Levis, R.J. and H.A. Rabitz, Closing the Loop on Bond Selective Chemistry Using Tailored Strong Field Laser Pulses. The Journal of Physical Chemistry A, 2002. 106(27): p. 6427-6444.

5. Warren, W.S., H. Rabitz, and M. Dahleh, Coherent Control of Quantum Dynamics: The Dream Is Alive. Science, 1993. 259(5101): p. 1581-1589.