A Promising Route to Compact and Economic Sub-15 fs, PW-Level Ti:Sapphire Lasers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a conceptual design for a compact and cost-effective sub-15 fs, petawatt-level Ti-sapphire (Ti:sa) laser system, utilizing a modified hybrid amplification approach. The design consists of a high-contrast broadband seed source (XPW+OPA), a dual-crystal OPCPA front-end, a double-grating Öffner stretcher, two-stage high-energy Ti:sa amplifiers, and a mismatched-grating compressor for passive fine dispersion management. The authors demonstrate a numerical analysis of this scheme, and simulation results indicate the feasibility of achieving a PW peak power with ∼13.3 fs pulse duration. This work offers significant insights and guidance for the development and upgrading of high-contrast ultrafast PW and even 10 PW level Ti:sa femtosecond lasers. So, I recommend this manuscript to publish in Photonics.

 

Some minor errors/problems should be discussed/clarified by the authors:

(1) P3: There are some minor error in Figure 1: 'TFP' has been mistakenly labeled as 'TEP'. It should be corrected.

(2) Line 148, the pulse cleaner icon should be drawn in Fig.1.

(3) Line 187, ‘using PVWC configuration’. ‘PVWC’ should be not used as abbreviation, because of the first appearance in context.

(4) P4: While the manuscript adeptly analyzes the feasibility of a new scheme for achieving sub-15 fs PW lasers, a critical aspect that warrants further discussion is the focusability of the compressed laser pulses. Given its significance in practical applications, it would be beneficial to add some descriptions of this aspect to comprehensively address the system's operational capabilities.

(5) P6: The manuscript describes the use of a polarization-encoded filter to suppress gain redshift. However, it's important to consider that the transmissive quartz crystal, as part of this setup, may introduce a post-pulse. This post-pulse has the potential to transform into a pre-pulse during subsequent amplification stages. Given the paramount importance of temporal contrast in high-field science applications, I recommend the authors extend their analysis to address the impact of quartz crystals on contrast. Specifically, strategies or methodologies to mitigate this impact would greatly enhance the paper's applicability and technical robustness.

 

 

 

Comments on the Quality of English Language

The quality of English Language is OK.

Author Response

Dear editors and reviewer,

Thanks to the reviewers for their constructive comments and suggestions, which are really helpful to improve the quality of our manuscript. Please find below our responses and revisions. The reviewers’ comments are Bold Italic characters, while our responses are Roman characters, the revisions of the manuscript have been highlighted in “red” in this letter. In addition, references and line numbers are in accordance with the revised manuscript.

    Best regards

       Yuxin Leng

  

 

Reviewer 1:

1) P3: There are some minor error in Figure 1: 'TFP' has been mistakenly labeled as 'TEP'. It should be corrected.

2) Line 148, the pulse cleaner icon should be drawn in Fig.1.

 

Response and revision

Thanks for the reviewer’s reminder, we have corrected it in Figure 1:

 

3) Line 187, ‘using PVWC configuration’. ‘PVWC’ should be not used as abbreviation, because of the first appearance in context.

 

Response and revision

Thanks for the reviewer’s suggestion, we have given the full name of PVWC in Line 188:

“. . . first and second BBO crystals by using Poynting vector walk-off compensation configuration. . . ”

 

 

4) P4: While the manuscript adeptly analyzes the feasibility of a new scheme for achieving sub-15 fs PW lasers, a critical aspect that warrants further discussion is the focusability of the compressed laser pulses. Given its significance in practical applications, it would be beneficial to add some descriptions of this aspect to comprehensively address the system's operational capabilities.

 

Response and revision

In addition to meticulously optimizing each component of the laser system, one of the prevalent approaches to enhance focusing capabilities in this field is the implementation of deformable mirrors. These mirrors offer dynamic adjustment of the wavefront, allowing for precise control over the beam focus. This method is particularly effective in compensating for aberrations and improving the quality of the focal spot, which is critical for the performance of ultra-intense and ultra-short laser systems.

Thanks for the reviewer’s suggestion, we have added some discussion in Line 125:

“. . . Beside, a deformable mirror is adopted to improve the focusing ability of laser pulses.”

 

 

5) P6: The manuscript describes the use of a polarization-encoded filter to suppress gain redshift. However, it's important to consider that the transmissive quartz crystal, as part of this setup, may introduce a post-pulse. This post-pulse has the potential to transform into a pre-pulse during subsequent amplification stages. Given the paramount importance of temporal contrast in high-field science applications, I recommend the authors extend their analysis to address the impact of quartz crystals on contrast. Specifically, strategies or methodologies to mitigate this impact would greatly enhance the paper's applicability and technical robustness.

 

Response and revision

Thanks for the reviewer’s suggestion, we have added some analysis in Line 207:

“. . . To avoid introduce post-pulse, this quartz crystal can be divided into two part with same wedge angle.”

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the authors present an innovative sub-15 femtosecond hybrid amplification scheme that simultaneously feature cost-effectiveness with a compact structure. This design replaces traditional regeneration and pre-amplifiers with dual-crystal OPCPA front-end, to achieve 1 PW power with just three amplifier stages. It has shorter output pulse duration than those of extant counterparts, which implies less energy is required for equivalent peak power, thus results in a significant reduction in the diameter of large-diameter components. This reduction not only saves costs but also contributes to the system's compactness. The proposed scheme offers insightful guidance for the construction of Ti: Sa based petawatt laser facilities. This work is interesting, so I recommend to publish this paper after following revisions are addressed.

1) In Line 93, the paper claims that “By carefully designing the non-collinear and phase-matching geometry, the gain profile of this OPCPA front-end will be spectrally compatible with the upstream high-contrast seed source and the downstream Ti:sa CPA chain”. Can you explain clearly what means “spectrally compatible” here? And I think it is better to omit “and” from the sentence to enhance readability and to avoid potential confusion.

2) In Line 200, it is noted that the traditional acousto-optic programmable dispersion filter is replaced by a PEF, which, while effective in spectral filtering, lacks flexible dispersion control. The manuscript mentions using a mismatched-grating compressor as a remedy. However, considering that this compressor is typically situated in a vacuum, making adjustments be challenging. A discussion on how to practically address this issue would add significant value to the paper, making it more applicable to practical scenarios.

3) Of course, for a given pulse power, shorter pulse duration needs lower pulse energy, which makes the setup be cost-effective and compact. However, shorter pulse duration means larger spectral bandwidth, which may result in a decrease of the laser gain, thus required more complicated amplifiers. Please give some comments on above influence.

 English shall be carefully checked, for example,

4) In Line 50, “are now operated” shall be “operate”.

5) In Line 70, “the spectral width of the seed source” is better to be “the seed bandwidth”.

6) In Line 75, “still can support” shall be “can still support”.

7) In Line 77, “near Fourier-transform-limited (FTL) compressed pulse duration” is better to be “the compressed pulse with near Fourier-transform-limited (FTL) duration”.

8) In Figure 1, the caption mentions a 'dichroic mirror (DM)', which is absent in the figure itself. Additionally, the label 'TEP' within the figure should be corrected to 'TFP'.

9) Regarding Line 137, the use of the abbreviation 'XPWG' seems unnecessary, as it is only referenced once throughout the manuscript.

10) In Line 144, “the spectral width of it” shall be “its spectral width”.

11) In Line 184, “amplified seed pulses energy” shall be “amplified pulses energy”.

12) In Line 225, “naming” shall be “named”. 

Comments on the Quality of English Language

 English shall be carefully checked and improved, for example,

1) In Line 50, “are now operated” shall be “operate”.

2) In Line 70, “the spectral width of the seed source” is better to be “the seed bandwidth”.

3) In Line 75, “still can support” shall be “can still support”.

4) In Line 77, “near Fourier-transform-limited (FTL) compressed pulse duration” is better to be “the compressed pulse with near Fourier-transform-limited (FTL) duration”.

5) In Figure 1, the caption mentions a 'dichroic mirror (DM)', which is absent in the figure itself. Additionally, the label 'TEP' within the figure should be corrected to 'TFP'.

6) Regarding Line 137, the use of the abbreviation 'XPWG' seems unnecessary, as it is only referenced once throughout the manuscript.

7) In Line 144, “the spectral width of it” shall be “its spectral width”.

8) In Line 184, “amplified seed pulses energy” shall be “amplified pulses energy”.

9) In Line 225, “naming” shall be “named”. 

Author Response

Dear editors and reviewer,

Thanks to the reviewers for their constructive comments and suggestions, which are really helpful to improve the quality of our manuscript. Please find below our responses and revisions. The reviewers’ comments are Bold Italic characters, while our responses are Roman characters, the revisions of the manuscript have been highlighted in “red” in this letter. In addition, references and line numbers are in accordance with the revised manuscript.

    Best regards

       Yuxin Leng

 

Reviewer 2:

1) In Line 93, the paper claims that “By carefully designing the non-collinear and phase-matching geometry, the gain profile of this OPCPA front-end will be spectrally compatible with the upstream high-contrast seed source and the downstream Ti:sa CPA chain”. Can you explain clearly what means “spectrally compatible” here? And I think it is better to omit “and” from the sentence to enhance readability and to avoid potential confusion.

 

Response and revision

The term 'spectrally compatible', as used in the context of Optical Parametric Chirped Pulse Amplification (OPCPA), refers to the requisite characteristics of the output spectrum. It's not only imperative that this spectrum is sufficiently broad, but it also needs to be as symmetrical as possible around the central wavelength of 800 nm. This symmetry is crucial to minimize the influences of gain narrowing and red-shift effects in the subsequent Chirped Pulse Amplification (CPA) amplifiers. Such a spectral configuration ensures optimal performance of the amplification process and is essential for maintaining the desired pulse characteristics throughout the amplification chain in ultra-intense laser systems.

Thanks for the reviewer’s suggestion, we have omitted “and” from the sentence in Line 93:

“. . . By carefully designing the non-collinear phase-matching geometry,. . .”

 

 

2) In Line 200, it is noted that the traditional acousto-optic programmable dispersion filter is replaced by a PEF, which, while effective in spectral filtering, lacks flexible dispersion control. The manuscript mentions using a mismatched-grating compressor as a remedy. However, considering that this compressor is typically situated in a vacuum, making adjustments be challenging. A discussion on how to practically address this issue would add significant value to the paper, making it more applicable to practical scenarios.

 

Response and revision

In response to the query regarding dispersion control in our laser system, I would like to clarify that adjusting the second-order dispersion in the compressor is effectively accomplished using an electronically controlled displacement stage within the vacuum compression chamber. This approach allows for precise and dynamic modulation of the dispersion characteristics. As for the management of third-order and higher-order dispersion, these can be meticulously adjusted within a specific range through fine-tuning of the stretcher. This method, which we detailed in our previous work [https://doi.org/10.1017/hpl.2022.29], provides a comprehensive approach to dispersion control, ensuring that our laser system maintains optimal pulse characteristics across a broad range of operational parameters. Such precise dispersion management is crucial for the performance and versatility of ultra-intense and ultra-short pulse laser systems.

Thanks for the reviewer’s suggestion, we have added some discussion about this aspect in Line 232:

“. . . Although the gratings in compressor is inconvenient to adjust when operating in vacuum, the precise control of dispersion can be achieved by fine-tuning the stretcher.”

 

 

3) Of course, for a given pulse power, shorter pulse duration needs lower pulse energy, which makes the setup be cost-effective and compact. However, shorter pulse duration means larger spectral bandwidth, which may result in a decrease of the laser gain, thus required more complicated amplifiers. Please give some comments on above influence.

 

Response and revision

1. Drawing from our extensive experience in constructing ultra-powerful and ultra-short laser systems, the proposed solution operates within a bandwidth range of 700-900 nm. This spectral range is well within the capabilities of current optical coating technologies. Consequently, key components such as mirrors, gratings, and crystals, essential for the system's functionality, are readily achievable with existing coating processes. This compatibility is crucial for the practical implementation and reliability of the proposed laser system.
2. To ensure efficient and broad spectrum amplification, our approach employs a dual-crystal Optical Parametric Chirped Pulse Amplification (OPCPA) front-end, optimized for phase-matching. This innovative configuration effectively replaces the traditional regenerative amplifiers and pre-amplifiers. By adopting this methodology, the necessity for Chirped Pulse Amplification (CPA) gain is markedly reduced, leading to a more streamlined and efficient amplification process. This advancement is significant in enhancing the overall performance and reducing the complexity of ultra-intense laser systems.

 

 

8) In Figure 1, the caption mentions a 'dichroic mirror (DM)', which is absent in the figure itself. Additionally, the label 'TEP' within the figure should be corrected to 'TFP'.

 

Response and revision

Thanks for the reviewer’s reminder, we have corrected the Figure 1 and its caption:

 

 

Comments on the Quality of English Language.

4) In Line 50, “are now operated” shall be “operate”.

5) In Line 70, “the spectral width of the seed source” is better to be “the seed bandwidth”.

6) In Line 75, “still can support” shall be “can still support”.

7) In Line 77, “near Fourier-transform-limited (FTL) compressed pulse duration” is better to be “the compressed pulse with near Fourier-transform-limited (FTL) duration”.

9) Regarding Line 137, the use of the abbreviation 'XPWG' seems unnecessary, as it is only referenced once throughout the manuscript.

10) In Line 144, “the spectral width of it” shall be “its spectral width”.

11) In Line 184, “amplified seed pulses energy” shall be “amplified pulses energy”.

12) In Line 225, “naming” shall be “named”.

 

Response and revision

Thanks for the reviewer’s suggestion, we have carefully checked and improved the English quality of our manuscript:

 

Line 50: “. . . are now operate. . .”

Line 70: “. . . the seed bandwidth. . .”

Line 75: “. . . can still support. . .”

Line 77: “. . . and eventually to achieve the compressed pulse with near Fourier-transform-limited (FTL) duration. . .”

Line 137: “. . . a cross-polarized wave generation stage. . .”

Line 145: “. . . its spectral width. . .”

Line 185: “. . . amplified pulses energy. . .”

Line 228: “. . . a passive dispersion control method named mismatched-grating compressor. . .”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Ultra-short pulse-width High peak power power laser were one of the hot spot researches, and this work proposed a 13.3 fs PW grade laser by numerical simulations, and their previous experimental and theoretical works strongly supported this numerical simulation. This work can offer a possible route for the development of compact and economic PW-level Ti:sa lasers. The work was interesting, and the achievement was significant. This work could be published after minor revision.

1.       In Fig. 1 Nd:YAG was used as pump source for OPCPA and Ti:sa amplifiers. in fact, it should be the second harmonic of Nd:YAG laser (532 nm) that was used pump source.

2.       In Fig. 2 the waveform of pump was proposed to be 6^th super-Gaussian distribution. As I know, waveform of Nd:YAG lasers (including its second harmonic) are normally Gaussian distribution. How much will Gaussian waveform of pump pulse affect the final pulse-width of this laser?

3.       In line 187 page 5, the first appearance of “PVWC configuration” should give the full name.

Comments on the Quality of English Language

Author Response

Dear editors and reviewer,

Thanks to the reviewers for their constructive comments and suggestions, which are really helpful to improve the quality of our manuscript. Please find below our responses and revisions. The reviewers’ comments are Bold Italic characters, while our responses are Roman characters, the revisions of the manuscript have been highlighted in “red” in this letter. In addition, references and line numbers are in accordance with the revised manuscript.

    Best regards

       Yuxin Leng

 

 

Reviewer 3:

1) In Fig. 1 Nd:YAG was used as pump source for OPCPA and Ti:sa amplifiers. in fact, it should be the second harmonic of Nd:YAG laser (532 nm) that was used pump source.

 

Response and revision

Thanks for the reviewer’s suggestion, we have added wavelength next to pump pulse energy to avoid misunderstandings in Figure 1:

 

 

2) In Fig. 2 the waveform of pump was proposed to be 6th super-Gaussian distribution. As I know, waveform of Nd:YAG lasers (including its second harmonic) are normally Gaussian distribution. How much will Gaussian waveform of pump pulse affect the final pulse-width of this laser?

 

Response and revision

Indeed, employing a Gaussian waveform in the amplification process typically results in only the central portion of the pump pulses being utilized for signal pulse amplification. This approach can lead to a substantial reduction in conversion efficiency due to the inherent characteristics of the Gaussian profile.

However, to address this limitation, we have explored the use of a Master Oscillator-Power Amplifier (MOPA) structure. This structure is designed to amplify a programmable fiber seed source, enabling the generation of a quasi-flat-top waveform. This method significantly improves the efficiency of the pulse amplification process. The feasibility and effectiveness of this approach have been successfully demonstrated in our SEL-100 PW front-end prototype. The quasi-flat-top waveform ensures more efficient use of the pump pulses, thereby enhancing the overall conversion efficiency – a critical factor in the performance of ultra-intense and ultra-short laser systems. Please see https://link.springer.com/article/10.1007/s00340-023-08000-3.

 

 

 

3) In line 187 page 5, the first appearance of “PVWC configuration” should give the full name.

 

Response and revision

Thanks for the reviewer’s reminder, we have given the full name of PVWC in Line 188:

“. . . first and second BBO crystals by using Poynting vector walk-off compensation configuration. . . ”

Author Response File: Author Response.pdf